Because they have no excuse, a jury would agree they should've known better.
Wait: you're arguing that doctors and researchers don't understand the risks of experimental treatments better than the general public?
Why would you think that?
Edit: It is much easier to break things you don't understand than fix them.
Because, usually, listening to a patient's great ideas is a waste of time. This of course is not the case, when the patient has provable knowledge that the doctors have to admit might surpass their own knowledge.
> [treatments] be totally experimental but facing the end most would take experimental over nothing.
Sure, except for that the relatives might sue a doctor after a patient's death. In court you have a much stronger position when the patient obviously had the knowledge to really understand the risks of some treatment.
Last but not least: Experimental therapies are much, much more expensive than standardized ones. You basically have to check all parameters permanently while with standardized therapies you just follow a protocol that dictates what to look out for.