This might be too much legalese and IANAL but they do have a majority market share in North America. That said, I'm not suggesting this is an easy lawsuit but that it's obviously an abuse of power over the ecosystem. That is, when your product (Apple Sign in) does not take off by itself you simply force the market to adopt it.

I'm no lawyer; so that may well be antitrust. But forcing their way in to the party isn't that similar to locking other offerings out of the ecosystem. It isn't easy to see how this hurts anyone strategically.

It'd be much more worrying and abusinve if they blocked users from logging in to certain products with their Apple ID.

>This might be too much legalese and IANAL but they do have a majority market share in North America.

Citation needed. They make the most profit, but do they make the most devices?

People seem to think antitrust violations means you have a monopoly, this is not the case. There are a whole host of antitrust rules beyond monopoly of the market.