Maybe it is just me, but if I want to buy a driver's shirt, I am going to buy a driver's shirt. If I want to buy the track's shirt, I am going to buy the track's shirt. It doesn't matter if the track refuses to let them set up in the stands, if I don't want to buy the track's merchandise then you're not going to entice me buy removing other options.
If the drivers were able to sell 10 shirts a night at $20 each then that is $200 in their pockets to help them come back next week and race. The promoter has already gotten the pit pass money from the driver and already gotten the general admission money from the fan. I think this promoter's decision is a little extreme. Why couldn't he charge them a percentage or something to make it a win win for both parties.

I've bought a lot of shirts from drivers and I've raced a lot, so I know the costs involved and how buying the shirts from the drivers can help with expenses. And if I know one thing, no drivers = no fans = no profits for promoter.
Just my opinion.