The association bonus gives you the established user privilege on a private beta. This does not really make sense in my opinion. Just joining a site doesn't mean that you are an established user.
I see two solutions:
- Increase the threshold for established user to, say, 150 reputation.
- Do not apply the association bonus to private betas until they reach public beta.
The first one might be the simpliest to do, and personally I would prefer this one.
But there is also something to say for the second point: the association bonus has been invented for giving basic privileges (in particular upvoting and commenting) to users who know how StackExchange works. However these basic privileges are already 1 rep on a private beta, so there is no real need for the association bonus.