Except for the fact that, you know, applying to a job in a union shop is a
voluntary act. Nobody forced you to apply to UPS, nobody forced me to apply to UPS, nobody forced
@Gumby to apply to UPS. Being in the union is nothing more than a condition for employment, because the company and the union agreed so. Not too much different than saying you need to pass a drug test before being hired, or that you need to produce your driving record, etc.. Don't want to take a drug test? Great. You don't have to, but you're not gonna get the job. Don't want to join the union? You don't have to, but look for work elsewhere.
See where I'm going with this? The draft, and having to join a union as a condition for working in a union shop are nothing alike.
Except in that case, coercion is involved. The protection offered by the mafia is from the mafia; it's extortion, plain and simple. To the extent that unions have been known for violence, blackmail, extortion, et al. in the past, the government should put a stop to it. But if a union and a company agree that all new employees performing a certain job (in the case of UPS, package handlers, drivers, and other similar positions) have to be a member of the union, they ought not to get involved.