Because unions often support policies that significant portions of the workforce do not like. If you're a teacher in America chances are your union dues are bei

Because unions often support policies that significant portions of the workforce do not like. If you're a teacher in America chances are your union dues are being used to oppose merit-based pay increases and retain seniority based pay. If you're a teacher who wants merit-based pay increases then you're being forced by the government to fund a group that is actively working against your own interests. Multiple unions would let workers join the union alined with their interest.