At the Seattle game I had tickets on the East side of the stadium, my wife and I went and sat down for a bit in an alcohol free section before the game, we were the only ones there......the usher came up to me and asked if we had tickets for that section, I told him no, our tickets were on the other side of the stadium.....he said "Ok, you can sit wherever you want, just so you know though, this section is alcohol free"
They don't care, as long as you aren't a dick to someone who's seats you are sitting in, just get up and move, I don't care if there are 30 empty seats around you that those people could sit in, they bought those tickets and have EVERY right to sit where they paid.
I saw some guys hastling an older couple this week over that very thing.......an usher took care of them.