Because A) this is good for consumers, and B) Apple doesn't have the market position to force consumers or other vendors to do anything. There is plenty of choice and competition in the smartphone market.

Apple forced nobody to implement this feature UNLESS they were already using another social media login option.

It is possibly an antitrust issue in an alternate universe in which our government still cares about antitrust issues, but since it's a net win for users, it might be a tough case to make.

Because no one has to use it. You can still use any other sign in option including good old email if you want.