change bb_capabilities in database

i’ve got bb_press integrated in wordpress, works fine, except that new registered users are missing bb_capabilities in the database and therefor can’t access the forums.

i’ve tried several places to add that, p.e. in registration.php by adding

update_user_meta( $user_id, 'bb_capabilities', 'a:1:{s:6:"member";b:1;}');

the problem is that it writes into the database instead:
s:23:"a:1:{s:6:"member";b:1;}";

and i just can’t figure out where that comes from and how i can make it add exactly what i need without modifying it.