When you're driving down the road and you let your pre-teen daughters lean out the car window and make nasty faces at people (especially those who are visibly upset) and you just sit and laugh and encourage them, don't be surprised when, one day, someone rams into your car or pulls a gun on your kids. I have a rage filter (albeit a shoddy one) but a lot of people don't. You're kids are going to learn that they're invincible and guess what? They're not. And it'll be your fault when their lack of manners and rude actions get them hurt or into trouble.
That is all.