If a union worker files for a return of part of his dues, objecting to the way they are spent, would he not become a target when layoffs come around? Might he find himself missing from the out of work list from which the hires are made? They do have ways to “get” you!
Since the employers and not the union would make the decisions regarding layoffs and call-backs, the union cannot impact this very effectively without it being easily visible and thus grounds for action from the member(the action as such would clearly be discriminatory based on belief and easily remedied). As far as the out of work list through the union halls, does it really matter? You're out of work! Either they will locate work as agreed or you'll take advantage of the union benes and financial support until they're broke.