Wow.. I think they can just say that they have special affiliate rules for certain situations through business dealings that exempt certain affilates from their normal blanket terms of service. I've seen TOS's that clearly state you can get in touch with the owners of the site before entering to attempt to work out their own personal TOS, but that it may not be successful.
My gut feeling is that it isn't this easy, and if sponsors want to accept the traffic, they can just change their terms to make sure they can operate how they want to. Why should they be liable? It's all about reducing liability.