Few people willingly pay car insurance; fewer still have any confidence that their insurance will pay-out. But we pay the premiums because it is a legal requirement, even though it might be more effective for some people to put that money into a bond instead.
It's a similar situation for unions; few people expect that a union really protect them from a determined private-sector employer, so they assess the cost-benefit of such "insurance" to be negative. Particularly when the premium, much as in your example, costs a lot in terms of minimum wage.