> Why is it that if someone wants to cut off their own healthy limbs, we are calling this a fetish rather than saying what it is, a mental illness.

I do call it a mental illness. If I came across someone that wants to amputate a healthy limb, I'd tell them they're mentally ill and need professional help. Most amputees I know feel the same way. As an amputee (RBKA, due to necrotizing fasciitis and sepsis), -I know a number of amputees and none have "wanted' to be an amputee.