I know this is old, but still pulls up as the result via a google search. So I'm going to post something I have had luck with.

If you know what sObject you are converting into, I've had success doing this:

objList //we'll say this is a Map<Id, sObject> where the sObject is an Opportunity
 List<Opportunity> oppList = objList.values();

Or looping:

   for(Opportunity o : objList.values())
   {
    //operations
   }

It's a little different than the previous answer given, so I hope someone finds it useful.

Edit: Apologies, realized this is for Map to Map. However I'll leave this because in some situations converting to a list from a Map will be beneficial as it should have already removed dups from being in a map, and then you can iterate through the list or perform DML on it.

What you've run into is a feature that's documented as Using SOQL Queries That Return One Record. Basically, there are some cases where you can assign a list that has a single item in it to a scalar value. Despite what the documentation says, you can use this in a variety of ways that are not specifically called out in the documentation, notably that this is a feature baked into the language itself, and not specifically to SOQL.

To understand what's going behind the scenes, there is an object that looks like a QueryResult. Apex uses this object transparently in order to facilitate the ability to behave the way it is documented. You'll see this in times when you JSON.serialize a query directly, and you'll find it has some specific behaviors. This object tends to not be null, and has a language-specific feature that allows you to access the first element if, and only if, exactly one item is in the list. It is also responsible for facilitating query cursors when there are too many results to fit in memory all at once.

As a simple example, let us write the following code:

Contact c = (Contact)JSON.deserialize(
    '{"Cases":{"done": false, "totalSize": 0, "records": []}}',
    Contact.class
);
Case[] cases = c.Cases;

When this code runs, we get the exception:

System.QueryException: Aggregate query has too many rows for direct assignment, use FOR loop

This happened because the underlying QueryResult object detected that the list was incomplete ("done": false).

Similarly, if we have exactly one entry, we can use the "Using SOQL Queries That Return One Record" behavior.

Contact c = (Contact)JSON.deserialize(
    '{"Cases":{"done": true, "totalSize": 1, "records": [{"Subject":"Need Help"}]}}',
    Contact.class
);
Case theCase = c.Cases;
System.debug(theCase.Subject); // Need Help

This code does not crash, as there is exactly one record to return. As long as the internal QueryResult meets the conditions that (1) done is true, (2) totalSize is 1, and (3) records contains exactly one record, you can use this behavior.

Interestingly, Map.values() appears to return a QueryResult object internally. This is inconsistent with the documentation for Map. However, I think this was documented this way intentionally, since we aren't necessarily meant to "know" what's going on in the backend. However, we know that this must be true, because the following code also throws the QueryException from above:

Map<Id, Case> cases = new Map<Id, Case>();
Case theCaseValues = cases.values();

If Map.values() did not return this QueryResult, then the language would not work according to its documented behavior. After all, we can't assign a normal list this way:

Case[] cases = new Case[0];
// Compiler error: Illegal assignment from List<Case> to Case
Case theCase = cases;

It is a shame that we only have the one page in the documentation that suggests this is possible. While it is not strictly "undocumented," I would say that it is poorly documented. Apex is working in the manner that it was documented in. Now that you know this feature exists, and has literally existed since at least the first public release of Apex, you can understand why it works.

I'd like to add that the Apex Language Server does not throw an error in VS Code when you write code that looks like this. The bug here is that any IDE that does not use the official Apex Language Server may be inconsistent with the actual implementation of Apex. Illuminated Cloud simply has a detail wrong about the underlying implementation, and I wouldn't blame them, as this is a rather niche construct.

Also, unless it is very explicitly proven to be a bug, which I don't think this is, this is more of a documentation omission. Salesforce R&D typically maintain that the behavior produced by the server is the correct behavior, and that the documentation is incorrect, instead.

That said, if you're going to use the behavior intentionally, you should leave a comment about how there's only ever one record returned, etc. You usually want to be more specific by selecting the first index, if that is your intent (someValues[0]).

You should not try to put a generic type in to a concrete type. This can cause bugs. The ideal situation is to create a copy so you retain the concrete type:

public Map<Id, SObject> removeLockedRecords(Map<Id, SObject> newMap) {
     // Make a copy //
     Map<Id, SObject> returnMap = newMap.clone();
     // Clear out existing key/values in copy (not original) //
     returnMap.clear();
     for (SObject s : newMap.values()) {
         if (!(Boolean)s.get('Locked__c')) {
             returnMap.put((Id)s.get('Id'), s);
        }
     }
     return returnMap;
}

By using clone, you preserve the original type that was passed in, and then your cast won't fail in the end.

One can use Map.containsKey() to check and initialise a list as indicated, or alternate way could be like this [matter of taste ;)]

Map<id,List<Entity__c>> mapProductEntity = new Map<id,List<Entity__c>>();
for(Entity__c et : [SELECT name,Field1__c, Product__c, Field2__c FROM Entity__c WHERE Product__c IN : Items ORDER BY Product__c ]) {
    List<Entity__c> entitiesForKey = mapProductEntity.get(et.Product__c);
    if (entitiesForKey == null) {
        entitiesForKey = new List<Entity__c>();
        mapProductEntity.put(et.Product__c, entitiesForKey);
    }    
    entitiesForKey.add(new Entity__c( field__c = et.field1__c,product__c = et.product__c, field2__c = et.value__c ));
}

This is certainly possible. You just need to build the map yourself instead of trying to create it form a a single query.

As an example lets say we have a custom lookup on the contact record to another contact, say 'Emergency Contact'. Its possible that a person is listed as the emergency contact for more 1 person, so we could create a map where the key is the contact id, and the value is actually a list of contacts in which they are the emergency contact for.

Obviously this is a bit different from your case, but the same idea and you should easily be able to adjust the code for your scenario.

Map<Id, List<Contact>> conMap = new Map<Id, List<Contact>>();
for(Contact parent : [Select Id, Name, (Select Id, Name From EmergencyContacts__r) From Contact]) {
    conMap.put(parent.Id, new list<Contact>());
    for(Contact child : parent.EmergencyContacts__r){
         conMap.get(parent.Id).add(child);
    } 
}
system.debug('My Map: ' + conMap);

You now have a map where a contact Id is the key, and a list of contacts that person is the emergency contact for is the value.

Hope that helps

EDIT

Thinking about it more, you could likely make trim this down a bit more and just do this as well.

Map<Id, List<Contact>> conMap = new Map<Id, List<Contact>>();
for(Contact parent : [Select Id, Name, (Select Id, Name From EmergencyContacts) From Contact]) {
    conMap.put(parent.Id, parent.EmergencyContacts__r);
}

system.debug('My Map: ' + conMap);

You're trying to put an instance of Account_Territory_Loader_vod__c into a list of Account_Territory_Loader_vod__c.

accATLMap.put(atl.account_vod__c,atl);

You need to instantiate a new list for each key in your map, and then add the element to the list:

// Check whether the value for that key exists
if (accATLMap.get(atl.account_vod__c) == null)
{
    // instantiate a new list
    accATLMap.put(atl.account_vod__c, new List <Account_Territory_Loader_vod__c> ());
}

// add the element to the list
accATLMap.get(atl.account_vod__c).add(atl);