Is there a rule for events that if you have good seats, you can't show up until the third or fourth fight in?
Always empty seats for the first few fights. 
No rule just that some people only show up a little before the main card 
Whenever I go to fights it is always like that. They don't want to be bored by the prelims because they have better things to do. people who buy the affordable seats get all the action they can.