It's the parents fault, nearly every time. The parents should make it clear to their children that disruptive behavior is not acceptable.
(I'm not saying it's NOT the parents' fault; it is certainly their fault in part, but the "how" and "why" of this is quite relevant.)
Many parents these days seem not to be teaching their children how to be polite to others, or how to act in public. And they tolerate disrespect from their children within their households. I would venture to say that parental influence is probably the overriding factor (along with the influence of one's environment and peers), on how children develop.
