PKM stated that security doesn't check backpacks at the beginning of a game, but during halftime. Hinrichs tried to get in before halftime, but during the second quarter, when the game was already in progress. Had he tried to enter with the crowd before the game started, it seems he would have gotten in.
And maybe that was the original plan, to get into the stadium before the game started, but for some reason he got there late. It makes more sense to me for him to have tried to get in with the crowds, instead of later on when he might be the only person trying to enter at a particular gate and consequently would get more individual scrutiny from security.
Anyway you look at it, he wasn't getting in with a backpack unsearched. It might have liquor in it.