I've often posted in favor of regulation against monopolies, but I'm not sure about this one.
I'm taking Call of Duty as an example, but the reasoning is the same for any other franchise. So, MS would have a monopoly on Call of Duty, but it's not like they'd have a monopoly on games, or even on FPSes. Activision/Blizzard owns a lot of franchises, that's true, but it's hardly a monopoly, even adding it to Microsoft. There are lots of companies making AAA games, and the indie scene is fairly healthy, and it's an industry where the barrier to entry is comparatively low.
Making CoD and a bunch of other stuff Xbox exclusive hardly means "forcing" gamers to use Microsoft platforms. There are other FPSes out there, and they are just as good, and if there aren't then there will be: the acquisition would do nothing to raise barriers to other developers, who would probably be happy to do their best to pick up Playstation gamers orphaned of CoD.
Sure, there are gamers that absolutely want to play CoD and specifically CoD and nothing but CoD, and I respect that but it just sounds like brand loyalty to me. Surely purchasing mere brand loyalty is not a monopoly action?
Compare this with the famous Windows wars: in the bad old days, you basically could not purchase any consumer PC without Microsoft getting their slice; any alternative channels were really rather difficult to get at for an average person. I don't know, but this really doesn't feel like the same kind of problem.