2001 President George W. Bush recently signed an executive order requiring posted notification to many workers that they don't have to finance the political activities of their labor unions. In 1992, then-President George H.W. Bush issued an executive order requiring federal contractors to post notices of what the Beck decision required. Within weeks of becoming president, Bill Clinton rescinded the order.
Unions devote considerable effort to political campaigns.
Unions have a right to pursue political goals, but non-members have a right not to be forced to subsidize that pursuit. The notices are simply reminders.