Ebisu and Tsunade also used the Shadow Clone technique and the Chunnin exam examiners recognized it (and were impressed that Naruto could use it). Ever since then I've been under the impression it is less a banned technique and more of a 'for high level ninja's who know what they're doing only' technique. If they taught it as an academy skill stupid kids would probably try to use them like Naruto does and drain their chakra to nothing very quickly.