That employer already entered a contract with another group of people, that lists them as a sole supplier of labour for said employer.

Imagine if I had a cabbage farm. I don't care about corporate politics, I just want Whole Foods to buy cabbages from me.

But they won't. They already have an exclusive supplier contract for cabbages with Dole, or Sunrise Produce, or whatever.

Is this unfair? I don't like it? That's my problem. I should not be demanding that there be a law that forbids sole suppliers, of cabbages, or labour.

If I still want to sell cabbages, I should talk to Dole, Sunrise Produce, or start my own grocery store. Or negotiate a new contract with Whole Foods, after their current contract expires.

Providing services to an organization isn't a god-given right. If you don't like the working environment, the requirements of the job, the pay, or the hoops you have to jump through to sell to a company, that's your problem. Go work for a non-union shop, or start your own company.

> They let two independent people make an employment agreement without anything interfering with them.

If one of those two people entered into an agreement with a third party, that lists the third party as a sole supplier of services for them, that's your problem. The law should allow that person to make such an agreement with said third party, to the detriment of you.

That's the libertarian response. They are bad because they stop people from entering into voluntary contracts.

The socialist argument is that they are bad because employment negotiation is not done on a person-to person level. It is done on an organization to person level. The organization has more power, and this power imbalance produces unfair contracts.

This imbalance is solved when you allow employees to negotiate as an organization. Right to work laws let scabs and freeloaders parasitically leech off the negotiations of others, without paying into the system that benefits them. The only way to deal with that is to exclude them.