Unions bargain for all the workers not just members. States have recognized and defended an agency fee for years. Why should non members be able to enjoy the benefits for free? Make no mistake, this is a Republican dream of union busting. Businesses band together in trade groups to lobby congress while the only voice a worker has is their union and no Republican will support union membership.
Do yourself a favor and research the Republican platform of 1956 and compare that to today's party of tRump.