Why are some sections of the stadium closed? What would be the harm in letting people sit where they want? It's a scrimmage, it's general admission, and the stadium is nowhere near capacity, so what's the big deal? Stupid regulatory randomness.
I was about to comment that repairs have closed sections in past games, and I was going to speculate that this was a potential reason.
Somewhat related, Bama drew approximately 92k fans for the debut of Saban as coach. That’s quite a few fans for a scrimmage.