> Libertarians and other free-market enthusiasts should actually be for collective bargaining and unions.
This depends on what you mean by "be for collective bargaining". As in most things, the details matter, and the slogan "be for unions" is almost meaningless.
In the sense that people should be able to form and/or join unions etc, then absolutely, I think most libertarians would be totally for it.
But often when people say you're for or against unions, they're talking about specific laws with specific consequences. For example, should an employer have the right to fire an employee who is attempting to start a union? A libertarian might well say that both sides have liberties that shouldn't be taken away in this case. If the employee manages to start a union and gets collective bargaining power - good for them! But the government shouldn't intrude on the employer's right to fire employees if it so chooses, no matter what the cause.
Another example - many countries have laws that say that if some percentage of the workforce, say 30%, agrees to form a union, then all employees are now automatically part of that union. This makes it much easier to form unions, obviously, and you might make the case that it helps protect the usually-less-powerful employees from the usually-more-powerful employer. On the other hand, this often means that individual employees don't have the freedom to negotiate their own specific terms with the employers, even if they prefer not to be part of the union. Or that the employees now must pay money into the union, whether they want to or not.
Again, all of these are legitimate debates and most of the sides of this can be defended. But the slogan "you should be for collective bargaining and unions" just doesn't include enough nuance to even being to address how you should react to each of the above - which are the specific laws that people are talking about when talking about unions.