Boob flashing (or nudity in general) is not common in the US and also isn't seen as okay or appropriate. It would be much more common in Europe, where views on nudity aren't as harsh.
Many American women and men see any flashing or public exposure/sex to be unclassy and it's illegal. Now, at places like parties, rock concerts, etc. Most of the people who attend are intoxicated with alcohol and sometimes drugs, lowering their inhibitions against nudity and other things. Also, (this is very severe generalizing) most people who go to such parties and concerts aren't very "classy" by most American's standards. They usually have more unorthodox lifestyles and wouldn't be phased by the stigma of indecent exposure that most American citizens despise. So that is why you see boob flashing in those videos.
Selective enforcement. It's typically pretty rare. Only happened to me once. Granted I'm not a party animal. I imagine if you go to places with lot's of drunk slutty girls your odds are better. But I'd be surprised if even you went to raves bars every weekend, you see a boob flash more than twice a year.
Pretty rare I'd imagine. Except for Mardi Gras in New Orleans. I've heard that can get pretty wild. Obviously, if you go somewhere people are likely to be highly intoxicated, the rate of flashing increases, but as far as I'm aware its relatively uncommon.
Its becoming more and more acceptable here in the US. Women can now breast feed their babies in public now too.
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It's pretty rare except for New Orleans around Mardi Gras. I don't see it on a regular basis.
It happens but I wouldn't say it's common.
It is illegal, in many places (which is dumb). Probably most common at parties, at Mardi Gras.
It's pretty rare. You might see a girl flashing at concerts, parties, and maybe certain bars, but for the most part you won't see a girl flash her boobs.
Those videos you've seen are call "Girls Gone Wild" and they are NOT the norm in the US.
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