Most hotels are not owned and operated by Hilton Hotel Corporation. W=A is. Franchisees are required to make rooms available for rewards. HHC gives themselves and exemption, just because they can.

I'm sure it has something to do with the fact that members of the Hilton family, including Paris and Nicky, have lived there on and off. Surely the family wants to keep it exclusive, and the idea of someone staying there on credit card points just doesn't cut the mustard.