This is for backwards-compatibility as the UTC object did not come into existence until (I believe) Python 2.7. The docs for utcnow() explicitly state that if you want a timezone-aware UTC datetime object that you should use now() w/ the UTC object passed in.

Now if you would like to have a keyword argument for utcnow() to cause it to return a UTC object (e.g., utcnow(aware=True)), that may be a patch that  could get accepted as it doesn't break backwards-compatibility if you leave the argument out.