I'm curious why you would want to opt out of a union in the first place, since unions generally double workers' wages. I live in the right to work state of Idaho, which has some of the lowest wages in the country, especially for things like farm work. I realize that this is a bit of a straw man argument though, which doesn't touch your main point.
I'm having trouble thinking of a case where a union charges more in fees than it provides in additional wages, benefits and other protections. So I think my main point is that you are fixated on a motivation that doesn't exist. It's like being angry that you must pay for a stamp to mail something through the post office, even though that costs a fraction of what UPS or FedEx cost. You're free to work somewhere else or use those other services.
Maybe someone else can answer this better than I can. I really do want to understand why unions are so controversial, because I've only experienced the downside of not having them. Like when I was moving furniture 20 years ago and the warehouse charged $34/hr and only paid us $10/hr, even though we were doing all of the work. My feeling is that had we been unionized, we would have made at least $17/hr.