The issue is as I stated I beleive. The user is placed in additional groups and what happens is if they belong to a group that has 0 free ads then they have to pay for ads. It is a check to ensure that if a user has payment etc that if they are part of multiple groups then the best possible scenerio is obtained and they wont have to pay have free etc.
If you want to set free ad based groups then you need to make real additonal usergroups in smf and resync them into classifieds and then set the users you wish into the other real usergroups not post based groups. See they still belong to the main usergroup that has no free ads therefore they will be prompted to pay.