I think they don't want to go through the trouble of opening it up to DVC members not staying at BLT. If they did that, how would those people get it? The resort is key-card access for guests, so that creates an issue of people waiting to get in, needing to hire a door attendant at entrance, people invariably wandering around outside and then not being able to get back in, etc. They could more easily open it up to any BLT guest, but I think they'd rather maintain the exclusivity as a selling point. (though it's a weak selling point IMO)
Of course, if it is financially unsuccessful, who knows what we'll see happen. (but again, I say it'd be easier to open it to BLT guests)