As described here: http://news.gmane.org/find-root.php?message_id=20150422115959.1ff2ee58%40limelight.wooz.org

Importing a submodule binds the submodule's name in the parent module's namespace.  This is surprising, but it seems intentional and it's relied upon by existing code, e.g. asyncio/__init__.py in the stdlib.

It's also not documented afaict.  It should be documented in the Language Reference's section on the import system.  After a little more discussion on import-sig, I plan on doing that.