Unlike the poetic post below by ckh23, I can respect your answer even though I disagree on a few points.
First, I misspoke by stating that 'they haven't been convicted'. What I meant was that many have not been charged, which is even more appalling. But, anyway, that doesn't really relate too much to the subject at hand.
What I don't understand is that you seem to equate involuntarily detainment of mentally ill patients with prisoners at Guantanamo Bay. The only relevant similarity between these two populations is that they are both being held against their will. What really matters when speaking of patient autonomy is mental capacity. Clearly, patients in psychiatric units don’t have mental capacity, but I would argue that the prisoners of Guantanamo do.
How do we know they have mental capacity? Well, they make understandably sound arguments as to why they’re participating in their hunger strike, and their reasoning remains consistent on a day-to-day basis despite being persuaded to change their behavior. Due to their present mental capacity, I think a better comparison would be the hospice patient who decides to stop eating due to unresolved pain, hopelessness, etc. What does the medical profession do in this situation? Well, typically, we let them die. Therefore, I believe we should allow the prisoners at Guantanamo to do the same.