Aren't exclusive contracts pretty common in business? There's a big difference between monopolizing the supply to a single customer, which happens all the time, and monopolizing an entire market.
But what's wrong with forcing the other employees to join to keep their jobs? That is fundamentally a requirement on the company, that they only hire people in the union. And that's no different from any other sort of exclusive contract. If a restaurant has an exclusive contract with Coke, is it sinister to say that Pepsi employees can't supply them, and they have to join Coke if they want to do that?
It's my understanding in some states in the US it's possible a worker will be forced to join a union if a certain number of other workers want a union, even if neither the worker nor the company hiring them consent to it. In about half of states under "right to work" though they do give employees the option to not join the union if they don't want to.
This happened to me when I was working a ~minimum wage job at a grocery store. At the time it was not a right to work state, and I was forced to join the UFCW. The union I was forced to join then made me pay dues, pushing me below minimum wage.
So yes the false premise here was it was just a voluntary contract, but it really wasn't since the law required them to act in good faith to come up with a contract with the union when they had no interest in doing so except for the violence of the state that forced them to. Otherwise they'd have just hired non-union workers.
I did explicitly ask them when I was hired that I absolutely did not want to join the union or pay the dues. The manager was happy to hear that because they didn't like the union either, he certainly would have hired me without requiring anything union related if not for the NLRA forcing the "good faith" negotiation blocking it.
it’s a free country—individuals have freedom to just work at a non union shop.