This reminds me of a concert I went to a while back of a very famous singer. I like to dance, but I'd never be the one to get up until most others do. The singer shouted out to the audience to get up on their feet and dance, so he could see everyone was having a good time. As a few rows in front of us were dancing and enjoying themselves, and all we could see were people's bums, not wanting to be the miserable ones, my friend and I joined in, only to be reprimanded by a guy in the row behind us! We apologised and sat back down to the view of people's bums! At that point, we turned and told him that we hadn't paid to look at people's bums either, but that the performer had told everyone to get on their feet, so that's what we were doing! He soon followed suit.
I do agree, that it's not the thing to do if most of the audience remain seated, but sometimes it's expected, even by the performer.