The will to be seen as an upstanding citizen is clearly not enough to support a general aura of politeness . . .. my guess is that as social groups become larger and larger, and repeat encounters between any two given individuals become rare, politeness takes a nosedive.

Imagine a local movie theatre which only your community attends: you see these people all the time. If you bring a cell phone in there and bother everyone, you'll have to take crap from them for the rest of the month.

Now think of a multi-plex to which a couple thousand people from all over a county flock every weekend. You don't know anyone in there and don't have to worry about hearing back from them should you commit some kind of trespass. Some people remain polite anyway, but it only takes one couple stupid and inconsiderate enough to bring crying baby to ruin it for everyone. Enter (so NYC says) the politeness police. Not that it will really work.