Presumably because instead of responding to the request for boobs.jpg with with an HTTP 404 error (meaning, “not found”), Iran’s censorship tech returns a 403 error (meaning basically "you are forbidden from accessing this resource).

The “boobs” are “forbidden” you see; the tech mirrors the ruling party’s moral stance, probably coincidentally. Trying the same curl command in Russia and China will likely just get you a 404 error, so the joke really only works with Iranian servers. The 404 version is slightly less funny: “We couldn’t find the boobs!”