Cartels (right word?) are illegal. They are when all the companies in an industry band together and stop competing, thus acting like a monopoly. The problem is they could raise prices and there would be no competition.
Compare this to unions, where all the workers in an industry band together to threaten companies. Why do unions work? Because they have a monopoly on the workforce -- if companies don't like the demands, too bad, there is no competition.
But unions aren't illegal like cartels. In fact, the left even worships them. I guess because the left hates business so much.
On a somewhat unrelated note, I saw a TV program crediting improved working conditions since the industrial revolution to advocates. sheesh! if the change was just advocates being listened to, then what would happen is a higher percent of budget would go to safety, and less wealth would be created. but we're hella rich today! why? because the overwhelming change was not from advocates, it was from everyone being richer and from better technology. working conditions were poor because we were poor, and there was a high demand for jobs, even bad ones. these people chose to work, and chose to send their kids to work for an extra dime. all the companies did was offer jobs people wanted.