"Intentional" is not perhaps the exactly right term, but it is working as expected.  There was a thread about this on python-dev ("PEP 3115 compliant dynamic type creation" and "adding types.build_class for 3.3"), that ultimately resulted in the addition of the 'new_class' function to the types module.  The documentation should probably just say that types doesn't handle metaclass __prepare__, and provide a link to the new_class function.