If a user chooses to sign up using Apple’s SSO, how are they supposed to also use your desktop or other apps if those don’t also support Apple SSO?
Whether or not it makes business sense to restrict the use of Apple ID elsewhere is a matter properly left to the app developer and not Apple.
I should have added - this is based on my own interpretation of the App Store guidelines. If I'm wrong, I'd like someone to point it out, since I don't want to be spreading misinformation.