Not all economists agree that cartels are bad. Also they are usually about prices for selling stuff, not for buying stuff (as in buying employees). Just because companies have agreements doesn't make it a cartel. It would be stupid if companies wouldn't be allowed to have agreements.

And as mentioned before, workers cartels (unions) seem to be legal in most "civilized" places.

Or think about another example: poaching somebodies wife. That also seems to be frowned upon in many places. Yet, it is just an agreement that nobody should make somebody else's wife a better offer.

People tend to forget with all those rules that they could also be on the receiving end of things. You may not be a worker forever, maybe one of these days you want to become independent and run your own company. It would be a shame if people effectively weren't allowed to do so anymore.

This is cartoonishly unreal... like it was written by an evil villain.