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.
Yes if it's just a voluntary contract I see nothing sinister. If some employees form a union that's not sinister. If the company signs a contract with them that's not sinister. But if some employees want a union and that automatically means they've forced the other employees to join rather than allowing the other employees to pick to work outside the union, or automatically means the company is involuntarily bound to contract, that would be a bit sinister.
Yes, but they expire in ways that unions don’t