Mass Effect was published by Microsoft before EA acquired Bioware. Everybody thought Microsoft alone had rights to the first game because they were the ones who published it. But in 2012 EA released the Trilogy with the first game included for PS3. Before that, PS3 never had Mass Effect (the first one) as far as I know.
This seems to be the same deal? Maybe Activision can release the trilogy on other platforms because they are bundled together? You can't buy the first Mass Effect standalone on PS3 (maybe on the PSStore?), probably because Microsoft still have the rights to the individual title? But if they are bundled together, it is basically another title. In this case, Sony might have all the rights to the individual games but not to the 'N Sane Trilogy, this allows Activision to do what they want with it?
I really don't know, this just reminded me of how the first Mass Effect never came to the PS3 until 2012 since it was published by MS :P