Few outdoor venues offer a natural bowl shape for watching a concert. That means that the people in back--who paid just as much to be there but couldn't get out of the house as quickly as the people closer to the front--can only see if everybody is sitting at the same level. That means that if you bring a standard sized beach chair to sit in, two things become apparent:
1. You are clearly more important than everyone else at the show.
2. You're lucky you got anyone to go to the concert with you. Sadly, it was someone who also thinks a 3-foot-high back is perfectly acceptable in the middle of general seating.