> If one company is exploiting their workers in a competitive market, what prevents those workers from going to work for any of the other companies?

Depends, is there a labor shortage or a surplus? It might be cheaper for a company to train a replacement than it is to treat employees better. If there's a labor surplus, then the employer has a lot of power of the situation.

> Slavery is a government regulation that says that if someone pays a stranger money then you have to do work you never agreed to do.

Nope. In fact, slavery was contract/property law. There wasn't a government regulation or statute that established or regulated it. That was part of the problem. Slavery was the ultimate in libertarian ideology because it recognized that through whatever means, individuals could end up the property of other individuals. It further recognized children as the property of their parents (and thus property of the slave owners).

You can consider indentured servants, for example. Someone willingly signs themselves into slavery to pay off the debt (usually the boat ride to america). Slavery was a natural extension of that concept.

The only role the government served in this situation was enforcing the slave contracts.

> They're an attempt to monopolize the labor market in an industry.

That's not a refutation. Libertarian ideology (particularly the free market form) has no problems with a monopoly.

I do, which is why I think government regulations and actions to break up monopolies is a good thing.

But in a market without government protection for workers, unions forming a labor monopoly is the only solution which can counteract the inherent power imbalance between employer and employee.

I'd not classify them as "abusive" because far more people benefit from strong employee protections than the people harmed by those protections. The ultimate harm is it makes businesses less profitable.