They are using their app store monopoly to gain a foothold in the single sign-on business. You could try to claim that there's no business there but look at how Microsoft got spanked for giving away a free browser. Now the Apple has over half the phones in the United States, I'm looking forward to them getting spanked very soon hopefully.
Oh? Microsoft got in trouble for using the Windows monopoly to 1.) gain market share for their free browser and 2.) forcing OEMs to not sell alternative operating systems so they could keep their Windows monopoly. You can read all about that here [0]
That's exactly the kind of behavior that Apple is exhibiting here.
It's pretty easy to come along and say that I'm "ignoring the basic facts of the case" though without presenting any facts of your own to back that up. So let's hear your take now.
They got in trouble for lock in. Their integration of Internet Explorer practically made out the de facto implementation and made Windows the de facto operating system on machines.
But again, Sign In with Apple sits in addition to the other offerings, it doesn’t replace them, and in no case are developers expected to be unable to offer regular sign in with email.
To my mind, you’re not wrong, but making a rather different argument.
If Apple is perfectly happy with other sign on, then privacy is certainly off the table. Otherwise, they wouldn't allow apps in their store offering any other sign on (extreme analogy would be apps with virus in it). Then why force developers to add Apple sign on even they don't want to, instead of just encouraging? They can easily do so by telling developers that "apps with no Apple sign on are allowed, but are placed low priority queue so the review process will take longer".
It is pretty standard "embrace, extend, and extinguish" strategy. Similar to what's happening in the browser market. They are waiting for majority of the apps to have their sign on and majority of users use Apple sign on, then update the policy to disallow other methods.
I mean I don't have any issue of Apple doing these in their own ecosystem, but at least don't be so hypocritical or naive when defending them.
So anybody who is using Sign In with Apple trusts Apple to do the right thing.
Besides, what monopoly? Apple is perfectly happy with other single sign ons — provided you also offer Sign In With Apple.