You've got it the wrong way round. You start by assuming everything is permitted and then you selectively (and ideally reluctantly) bring the weight of law to bear when you discover that it's a net benefit to society. It's a correction for gross inequalities of power.

That's something the libertarian left and libertarian right surely agree on - minimising the application of the monopoly of force and all that.

So you assume both employers and employees can freely organize and associate. That's your starting point.

Now - we've decided that the right to join a union should be protected (it evens out an existing power imbalance) and that collusion to force down wages should be illegal (because it amplifies an existing power imbalance to the detriment of society at large)

But we have to realise that the common situation here is a corporation against a single individual. How is it comparable to a cartel of corporations?