The problem is that not all prospective employees want the union to negotiate on their behalf, but as Motley noted, they currently have no choice.

As for your "you don't have to work there" argument, it is patently ridiculous and you know it. Arguing that people who don't want to be union-represented should simply steer clear of certain employers or sectors is like saying women should just steer clear of employers that want to pay them less. The reality is that all people should be free to choose the employment they want without having unfair arbitrary conditions forced upon them by some entity with political power, whether that entity is a union or an employer.