There used to be a rule for the Mac App Store (I don't know if it is there anymore) that forbade developers from selling the same app at a lower price somewhere else. Wouldn't this be a case of anti-competitive price-fixing?
What do I mean by that? Gatekeeper.
Yes they can and do regularly. The extreme example is of course an exclusive contract where the vendor agrees to make it impossible to buy the good elsewhere.
Apple, on the other hand, is imposing restrictions on which price you must sell it to third parties, and, that, I think, is illegal.
There are also other considerations. A company may get away with demanding certain exclusivity rules for a few years but now we are getting to the point of 10 years so courts will look at the situation differently.