vary from time to time

The legal regime that cryonics has operated under has been reasonably stable for the past thirtyish years, with some minor quibbles about registering as a cemetery or not. What reasons lead you to believe that the relevant laws will undergo any more significant changes in the next thirtyish years?

what obligates Alcor to keep their bodies frozen?

At least in part, the fact that the directors are also members, and desire for their own bodies to be kept frozen after they die.

You can already will your body to medical research.

Legally, that's essentially what the wills of cryonicists already do. (In Ontario, the relevant statute is the 'Trillium Gift of Life Act'.)