If spammers can just create their own provider, that would kind of remove the point of having IDPs act as a spam gate. That's not a firm "no" but like, you are

If spammers can just create their own provider, that would kind of remove the point of having IDPs act as a spam gate. That's not a firm "no" but like, you are transferring the user-reputation problem into an IDP-reputation one, so allowing arbitrary IDPs is not going to work. If you allow federated ones, you need some whitelist or reputation network of which federated IDPs are the good ones.