I like to be around people who don't constantly emphasize their every word, making it hard to tell when something is actually important. Since swearing is a verbal marker of importance, its casual overuse is like shouting all the time; it's very wearying. And, lest I be accused of rationalising, I do not only apply this to children, but have also asked my wife to cut back on swearing.

As a side note, Americans are very loud, both in the literal sense of putting more decibels behind their voices, and in their over-reliance on swearing. I think you've fallen into the bad equilibrium that comes about when everyone has an incentive to be a little louder than the next guy, and there's no cost to being so.