That's fine if you're prepared to trust them as a kind of root authority. But it's not always appropriate. By sending an email and having the user visit the act

That's fine if you're prepared to trust them as a kind of root authority. But it's not always appropriate. By sending an email and having the user visit the activation link you confirm with fairly high certainty that they are in possession of the email being used. I don't think some of these alleged identity providers should have as much power and trust as they have.