Correct me if I am mistaking. Thai kids (I was told) are taught little rhymed poems among which one is : a kick can be easy countered by a punch, a punch is countered by an elbow, an elbow by a knee, a knee by a throw. It looks to me that in the logic of the game, it was allowed to sweep the leg of the opponent who is careless in attempting to knee and it was a part of the game as much as it is legitimate to consider elbows not only very important in themselves but as a part of the countering arsenal against an agressive boxer. Same for sweeps - with the correction that the difference is that, being not strikes in themselves, they are limited in respect to conserving the "striking art" character of the ring game, whilst being accepted as a part of the arsenal able to increase quality and technical difficulty of the arsenal without depriving the show of its main quality.

Just an outsider's opinion. I am curious whether there are such interpretations of otherwise illogical rules.