What constitutes foul language may be cultural but that's beside the issue. Imagine, a child from 1950s American suburbs was raised in that belief that certain words were wrong to use. The child was consistently reinforced in this belief throughout his childhood by all adults he met but never had it explained to him. Later, as an adult, he finds many friends who swear freely, and, not knowing the reason it is supposedly wrong, adopts the practice himself. Yet, in the back of his mind he still remembers his entire childhood telling him it was wrong and he know that those people genuinely believed that it was wrong. Clearly there was a reason for this taboo to begin, but without knowledge of the reason, how is he supposed to know whether it is still relevant? It doesn't matter whether there is any intrinsic property of the words that is wrong, what matters is thet there is a taboo and whose to say that the taboo isn't still relevant?
Teaching kids not to swear as a rule is especially important when one remembers that there are still places in society where the taboo holds strong and for the children to succeed in those places, they must not accidentally say anything that will offend someone.