They do but liability waivers don't absolve the producers from any responsibility whatsoever. Once they are aware of imminent physical danger to the contestant they are pretty much required to act or may be found to be grossly negligent which overrides any liability waiver. Not to mention the doctors own ethical responsibility. And we haven't even started on the whole insurance discussion.

Mostly, I don't think most people would feel comfortable watching a show where people are coming away with amputated limbs or whatever permanent injury/issue so they can win some trivial amount of money (relatively speaking). At least I hope not. I'm not even a huge fan of the 'set up to fail' circumstances they are put in so it ends up being a starvation contest for most, but I understand that they are doing that for a reason not just for the sadistic aspects.