I've been thinking about this and I believe the biggest issue that they would run into they charged everyone for the event is the way in which BGW marketed the memberships, when they first rolled out. Unlike the passes, which did not include (in the case of Williamsburg) Christmas Town, the memberships were supposed to give holders access year-round to all BGW activities. Dan described it as buying in to a club that gave you exclusive access. That is why you pay every month: you are a member, not a pass holder.
While I think the SWSD event is a great model in theory, I believe it would violate the spirit of the Williamsburg membership program. Who knows if the current team at BGW understands the concept behind the switch from passes to memberships; perhaps they mistakenly view it as simple re-branding. Either way, I think replicating it at BGW would probably further frustrate members and violate the principles behind the program, itself.