Because the AMA isn't a true sancioning body in the sense you're thinking of. No one could show up in the stands, and the AMA gets paid.
Why? Because they get all the "back gate" fees, plus a promotor fee. Back gate fees are rider/crew entries, anyone wanting to race or be associated with a team, that money goes to the AMA. So, the more people they pack on the grid, the more money they make.
Conversely, when their show doesn't air on TV when/where it's supposed to be (like here in LA, it was supposed to be on ESPN at 2:30 but wasn't, though maybe it was a misprinting error beyond their control), or when no one shows up to watch, which is pretty much always the case, the AMA doesn't lose out.
Note, too, if memory serves, the AMA releases total gate numbers as attendence. So if you buy a ticked, go in Friday, leave, come back Saturday and Sunday, you get counted three times.