Most people claiming to be "pro free market" still oppose cartels. A cartel is the banding together of multiple companies or parties in an agreement to work together to raise prices.

A union is a cartel. I don't mean that in as a normative statement. Unions may still be worth having, even if they are a cartel in that they workers band together to raise what they charge for their labor.

But, that is the logic by which many "free market" advocates oppose unions.