Ridiculous logic games like this are a pretty good reason why excessive legalese is mocked in mainstream culture - because it is totally disconnected from reality and enthralled with their own existence.
There's still some role of autonomy here. Even as a prisoner he is allowed to determine what he may read, may do (within restriction), when he goes to the bathroom. Saying "No, you have no right to determine what medical treatment you do or do not receive", to me, undermines a civilized society, even for the incarcerated.
"Sorry you were diagnosed with amazingly painful metastasizing bone cancer. If you should go into cardiac arrest, be advised, though, we will not let you pass - we are going to work to keep you alive. This may result in brain damage to you. But so be it. It may result in long, expensive and painful procedures for the rest of your life. But so be it".
Note that this is different to, say, hunger strikes and force feeding, but instead about medical conditions.
Should they keep him on life support for fifty plus years, in theory, just so he can "serve out his life sentence" to the satisfaction of the state, for example?
From a medical provider's perspective, and similarly, someone with custody over another, ignoring and actively violating their medical treatment wishes (in my state, the POLST form - Physician's Orders for Life Sustaining Treatments, which has replaced and expanded upon DNRs) is just as much assault and battery as anything else, and may also be considered negligent and malpractice.
Didn't they fail that when simple kidney stones evolved to a life threatening condition?
I'm not doctor, and I don't know the case, but it wouldn't be first time a "correctional" facility violated this duty.
It's not a far fetch argument to suggest that incarceration in US prisons is cruel -- I'm suspect history will look back at us with horror.