Sharing code is pretty central to Github's public repositories, so they're in a different position than, for example, flickr or facebook, where other users are primarily consumers of your creative works.

That being said, the license grant Github asks for ("use, display, perform, and reproduce") seems to be a minimal set that is part of any OSS license anyway. It doesn't elaborate on any terms of such a license, but from the context it's clear that GPL/BSD/CC/etc. are considered acceptable (see for example the mention of "granting additional rights via license" which links to an overview of these licenses).