AFAIK Chris Pearson designed & released Cutline under the GPL for Tubetorial, which was then bought by SplashPress Media. SplashPress then decided to release Cutline under a non-GPL compliant license (Attribution-ShareAlike 2.5).

I asked Matt about this on the Syn-thesis thread. His reply was:

It used to be GPL, they changed the license — regardless we’ve always had attribution on it. We’re replacing it anyway, don’t want any of his junk touching our sites.

It wouldn't make sense to have a non-GPL theme available on WordPress.com, given all that's happened..