In MMA, it is a "mix" of "martial arts". The things that are currently banned in MMA are generally also banned in other martial arts. Other combat sport that allows leg kicks, muay thai and kickboxing, allow the type of kicks you are talking about. To ban the kicks you are talking about in MMA would mean it is no longer 'mixed' martial arts. It would be it's own martial art, with it's own set of rules for striking. Your original post, didn't specify MMA. If were thinking they should be banned in muay thai, then that sport would no stop existing.