Personally, I think its because we have to set a good example in everything!
So, why is it so bad when an adult is caught swearing in front of a child. Some people make it a BIG issue, almost the same as abusing a child and then say "You are not fit to be a parent".
I can pretty much count off about a thousand things that would be more important than not swearing in front of kids who are probably swearing when you have your back to them.
In the grand list of human behaviors that concern me, torture and genocide are near the top. Swearing in front of children ranks nearly last, right above men wearing ladies' undies in their bedrooms.
I generally believe swearing is not a good thing.