The current code makes sense to me and also mirrors the actual implementation of setdefault() for regular dictionaries. 

While I can see what you're trying do, it is a feature that __getitem__() is not called.  And for regular dicts, that is the only way to make it an atomic operation.

In any case, the proposal would be a behavior change that breaks code — users are likely and justifiably relying on the actual default value being returned rather than a possibly new value computed by the __getitem__() method.

There is also an efficiency concern.  Changing the behavior implies that missing keys would trigger one initial search, then a store for the default, and then another subsequent lookup.  Speaking only for myself, I know that I would never want that by default.  If required, I would prefer to override setdefault() and explicitly spell-out the new behavior.