That's the intended and documented behaviour, see https://docs.python.org/3/library/inspect.html#inspect.BoundArguments.arguments.

You can easily implement the functionality you need by iterating through Signature.parameters and copying defaults to the BoundArguments.arguments mapping. There is no need to complicate the API with a dedicated method for that (if anything, in 3.5 you can subclass Signature and use from_callable to have any functionality you want).

Closing this one as 'not a bug'.