Not sure what to sure what to say Jessica, except welcome to LA shows.
I understand your frustration, feelings and overall sh*tty experience. I agree that you should not have to sit if you don't want to at show, and you made the right call moving, even though you had no real reason to move. At Hollywood Bowl in 2007, I saw entire sections sitting for the whole show both nights. Those were some of the best shows too. People in LA want to be entertained, they won't get up and have fun unless they know the song. Hence the hit/radio friendly shows typical of LA area shows.
To let you know that you weren't alone in your bewilderment of people sitting on Sunday night. When Squirm started, it seemed like the entire venue sat down. Orch, loge, terrace, lawn, everyone. There were even people in Loge 5, where I was, that sat all night. I am not surprised by this anymore, seeing that I see DMB pretty much only here in Socal. But I don't let it ruin my concert time. If you want to sit, by all means sit. That is their choice, but it is not fun nor comfortable to be threatened for not sitting. So I understand your statement about not wanting to return to Irvine. If you change your mind as the days, weeks, and months pass, maybe think about getting seats next time or hooking up with some local Ants to maybe make your next visit more pleasurable.