Hi,
I'm not sure what the problem is.
As long as the sanction, "C" if I remember, lists any restrictions, they can put any restrictions on their event that they want. It's done all the time; AMA sanctions an event but the participants must be IMAA members, or only warbirds are allowed to fly, IMAC, etc.
As far as the 50 mile rule, they want people to join and pay dues, and not get to use the field for "free". However, anyone not wanting to join can get a permit and still fly at the field, as I understand your post, so what's the issue?
BRG,
Jon