The buck has to stop at the top. That boy isn't cussing up a storm if the adults in the room aren't shining a very embarrassing spotlight on him. They seem very comfortable using him like a prop for their own amusement. You can't treat people like a spectacle and then act all
when that negative energy gets transferred back your way.
And y'all are right that our parents wouldn't have tolerated their kids using profanity like that. I just hope it's mainly because they would be actual adults and wouldn't have put their kids in such an embarrassing position then just laughed at your annoyance (that they created) in the first place. Not because "You're my child and your gonna take this abuse or else."