That's true but apparently the RC field or AMA got permission from the FAA for their field. No way would AMA allow a field to fly in a spot that the FAA disallowed. Their insurance wouldn't allow it.
I'm sure that they do have an arrangement, and I'm sure that it comes with very stringent rules. That doesn't change the airspace though, and anyone could fly there, right on the runway centerline, without any affiliation to the field and without following any of the rules that they presumably follow.
Back to your statement "In many cases DJI restricts flights in airspace that is wide open." I still haven't seen any examples.