historian said:
Whether or not they are legally labeled as cartels, it seems pretty clear that they are.
"Cartel", of course, brings to mind drug empires. As applied to labor law, it's an entity or group of entities that work together to manipulate prices or services without allowing fair competition.
If there's 50 vendors for a given product or something, or it's an easy market to get in to, you can usually rely on free market forces to set the value. If the value is set incorrectly, one of the entities will offer the product or services at a better deal. The rest of the participants will have to follow that lead or lose share.
When there are few (or one) participants, and there's a huge barrier to entry, then it could occur that the few entities talk among themselves and agree to set the price of a product at a rate guaranteeing a certain profit, or only hire people at a certain rate to prevent those hirees from getting a better deal elsewhere. This is anti-competitive. The entities involved have formed an "illegal cartel" in the eyes of the law.