I think there is a really good reason for telling kids not to swear, and it's the same reason novice speakers of a foreign language are warned to avoid swearing until they're absolutely, positively certain they understand all of the subtleties involved. To go into a foreign culture throwing around swear words willy-nilly is to risk using them in the wrong context or in the wrong company, which can make one a social outcast or even the victim of violence. One can never get into trouble for not swearing, and with the cost of incorrect swearing so high, the best course of action for most novices is not swearing at all.

Likewise, as novices, kids lack the experience to know when the use of swear words is or isn't appropriate, and in such a case, the safest course of action is to avoid using them altogether.