That authorisation is an assertion of identity. This makes OAuth an identity system inasmuch as WS-Federation and the SAML protocol are identity systems. Technically they're parts of an identity system, making them identity meta-systems.
Maybe, but that has nothing to do with OAuth. One could be completely within the OAuth spec and allow one user to provide grants to act as dozens of other users. You are basically saying that OAuth is not good at something it was never designed to have any say about.
It's as though you're saying that your rolling pin doesn't work well as a meat tenderizer. Yeah, I guess they're related, but that's not what a rolling pin does.
