People are decapitated, because they're the direct object of "decapitation" – the verb describing the removal of the head from the body.
I disagree that "we mostly consider the head when talking about people". We might mostly consider the face when talking about identity, but what specifically we consider in a person is context-dependent.
Also, "decorporation" would indeed suggest the removal of the body – but from what? "Decapitation" makes sense because the head is a part of the body. "Decorporation" would require the body to be removed from a larger whole, if it's to follow the same latinate prefix pattern.
Indeed, to remove the body from the important part — the head.
Likewise a “head transplant” always seems like a weird thing to discuss. It’s really (from my PoV) a “body transplant.” Unless you’d be happy receiving a head transplant from me :-)
OK, I reconsidered slightly my response to you (but can’t edit any more). The construct “a decapitated body” doesn’t sound odd to me, but “He was decapitated” does sound odd.
Historically the soul was seen sitting in the heart or the diaphragm, the chest basically, as far as I know. Because the torso exhibits features looking like a face (nipples as eyes and so on) which works to scare animals, at least. Therefore, as far as there is a concept, its should be stronly tied to the torso, from a developmental perspective. Decapitation is a rather archaic practice, anyway.
I disagree that "we mostly consider the head when talking about people". We might mostly consider the face when talking about identity, but what specifically we consider in a person is context-dependent.
Also, "decorporation" would indeed suggest the removal of the body – but from what? "Decapitation" makes sense because the head is a part of the body. "Decorporation" would require the body to be removed from a larger whole, if it's to follow the same latinate prefix pattern.