BuddyPress.org

This is a known feature of the wonderful and simple (sarcasm intended) bbPress and wpmu integration mechanism.

Try this:

http://michaeleagan.com/plugins/wpmu-enable-bbpress-capabilities/

This goodie enables new wpmu users as ‘members’ instead of ‘inactive’. You’ll have to go through and edit all existing users to take ’em out of the ‘inactive’ bucket.