A very good question. It could be that specific "humane" methods are mandated, or there might be specific criteria as to circumstance which might proscribe applying it to some situations that might warrant it (perhaps there are requirements of significant, imminent threat that early or middle stages of a degenerative cognitive ailment fail to meet, while one might prefer to be preserved while still more functional depending just how one judges the odds).
I am not sure what the actual legal landscape looks like.
I'm reposting this from HN's front page, because it brought up a non-cached thought on cryonics:
In short, end-of-life medical care is often pointless, painful and costly; doctors and ER personnel know this so well that they go to great lengths to ensure it doesn't happen to them.
It seems as if our systems and conventions around end of life are designed to not let people have a say in how they spend their final moments, even when letting them have their way would result in significant savings (note the dollar figures quoted above). I've already speculated on why that might be, but I keep seeing that turn up in unexpected ways.
I suspect that this is the bigger obstacle to cryonics, not so much e.g. the lack of scientific proof. "Freeze me cheaply instead of spending insane amounts of money on brutal attempts at keeping me alive" sounds like a sensible thing to tattoo on your chest, but the evidence suggests that it wouldn't be honored any more than "DNR" tattoos.