I think amputation of a non-diseased limb is probably seen as doing harm. The lack of motor control of a limb is a deficiency of the limb, but it doesn’t rise to the level of being a health threat like gangrene or risk the rest of the body. Amputation doesn’t cure the lack of motor control.
And in time, with intensive physical therapy, some “permanently” paralyzed people get some kind of motor control back. Plus there’s always the very rare chance you might get some kind of spontaneous or unexplainable recovery – a lot of paralysis is the result of trauma and healing can be time consuming, amputation just guarantees you won’t have it.
Plus amputation has its own risks since you use general anesthesia. And it has non-trivial side effects, like phantom limb pain. I’ll bet losing an arm completely plays havoc with your balance to some degree; even if you can’t use a dead arm its mass means something.
I would say most surgeons would only agree to do it with extensive review of the patient’s history, the details of their condition and maybe even a psychological evaluation to make sure they are not suffering from depression or some other mental illness that would cause them to want to mutilate their body.
I can see if you were a military veteran with a war wound, say an arm with disfiguring severe burns, catastrophic nerve damage and shattered and unusable elbow joint that resulted in the arm being “very unattractive” and a social impediment, uncontrollable and unusable due to the joint problem they might consider it.
It’s a strange idea if you think about it. Nobody advocates digging the eyes out of blind people or cutting the ears off deaf people or cutting the noses off people who can’t smell, why amputate someone because they can’t walk?