It's not free - you're agreeing for them to track and advertise to your customers. Apple does not do that with your Apple login data.
https://www.apple.com/legal/privacy/data/en/sign-in-with-app...
>When you use Sign in with Apple on Safari, Apple sees when you sign in to a website so that Apple can authenticate your sign in, but Apple does not retain a history of what websites you sign in to or when you use Sign in with Apple.