Not only does the license allow for this, but aren't the people doing the fork for Woothemes the core authors of Jigoshop?
So yes and yes. The comments actually clarify the situation a little bit beyond that (apparently this fork was their last resort):
http://www.woothemes.com/2011/08/the-good-stuff-new-team-mem...
And the prospect of forking was mentioned throughout any offers, which were not close to being acceptable for a variety of reasons - http://jigoshop.com/blog/2011/08/26/our-forking-views/
We've lost awesome, awesome people from Matasano to better paid roles elsewhere; some of those people even compete with us now. We're not going to whine about it. I'm glad I got to work with them at all.