Sim locking is purposely breaking the normal functionality of a device in order to limit competition.
I wouldn't care so much if there were plenty of other affordable ways to get a phone and coverage, but sim locking is so prevalent in the US that it's unavoidable in many cases, and most phones you can get unlocked are incredibly expensive.
OK, thanks. Now please explain how any of these companies fall under antitrust definitions.
(I'll save you the effort: they don't. None have sufficient market share that anyone would go after them and there are enough BYO provides to look at that there's no way you'll win a claim you don't have choice.)