Because the assumption here is that the request and the response are both in good faith. If the DNS operator blocks requests, this good faith expects that I will respect those wishes, not that I'll then try to circumvent those wishes. Spammers, on the other hand, not only take do attack servers to discover and exploit vulnerabilities, but like botnets, they attack other third party machines to circumvent server blocked requests/throttling. Additionally, most such DNS requests are done singly, whereas most spam is dumped by the billion.