What country are you from? In the US most people who can afford it have more than the legally-required car insurance and insurance companies are expected to pay out in the event of an accident.
A bond isn't insurance because, on average, it won't cover your expenses (if you earn enough on it to pay for a catastrophe, that means you didn't need it). The whole point of insurance (and unions) is to share risk over a large number of people, only some (random) fraction of whom will ever "need" the insurance.
Regarding unions, it's first and foremost not a question of protection from a determined employer, but protection from specific parts of the company doing illegal and/or wrong things.
As soon as your company has more than 10 employees, it gets a whole lot more complex than "employer" and "employee".
Actual experience has borne out these expectations.
I remember one jokey comment that low pay in prospect terms means some one who isn't paying any higher rate tax :-)
By that rationale, no-one would by the more expensive fully-comprehensive insurance (where your own car is insured, not just third parties) but they do.
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.