I'm sure it's rare, but, then, I used to use foul language with clients. And the reason was rather as suggested above. It's the only language they understand; they use the F and the C words in their own everyday speech, the F word being a favourite intensifier. Saying "I say young man , don't behave in such an unseemly manner" is hardly going to work !
And it's true that mere foul language, without being part of a pattern of causing alarm etc or a general breach of the peace is not sufficient for an arrest. A long time ago it was, and the fixed penalty for such 'vulgar abuse' was graded according to the status of the offender, but we've moved on since the days of yeomen, serfs et al.