Since this is the 40th, I think it might be reasonable to assume that a DVD is in the offing -
as to "union rules," does anyone know if this "banning" is a precedent, or have they ever done it at, say, a Trek convention? And if so, for what reason? A gathering of fans and former stars of an older tv show doesn't strike me as a "performance" under standard union rules - nor is the organization under which the fests are organized considered a "union house" under which AFTRA or SAG rules would normally be considered in force (although IATSE rules may be in effect in certain venues for purposes of the union technical support which might be needed to mount an event).
In the case of a live performance of, say, a copyrighted script using AEA stage actors(actors equity), it is customary not to film or photograph without union, copyright holder or production company permission. The creator or writer may make this request solely, as well. However, if union rules were to be in force in a situation like the fest (highly doubtful), then the "performers" or guests would be owed union scale fees, at the least. There would be contracts, etc to sign. Being a SAG or AFTRA or AEA performer does protect you in terms of fair use of a performance(getting paid, etc.) - but I haven't heard of the definition of performances being extended to autograph lines, informal panels (talk shows do pay scale to union performers) or Q & A's. It would certainly not apply to unrehearsed interaction between fans and guests or fan "performances" and costume parades.
I think more information is needed on the whys and wherefores of this particular situation before drawing conclusions, but, left to guesses, its probably about exclusivity for a later DVD release, or an attempt to protect a few segments of the program or both. In any case, union involvement is unlikely - intellectual property or copyright concerns would be a little more plausable.
Petofi