I'm not sure if you're trolling or not, so I'll assume naiveté and give you a thoughtful answer: Because the resource you tried to delete will not be deleted. To add insult to injury, from a UI perspective, it will still behave like it has been. This wrecks any ajax applications that cache GET requests.
Furthermore, it's in direct violation of the HTTP spec:
> If the request passes through a cache and the Request-URI identifies one or more currently cached entities, those entries SHOULD be treated as stale. Responses to this method are not cacheable.
Furthermore, it's in direct violation of the HTTP spec:
> If the request passes through a cache and the Request-URI identifies one or more currently cached entities, those entries SHOULD be treated as stale. Responses to this method are not cacheable.
[http://www.ietf.org/rfc/rfc2616.txt]