Yeah, I never understood how "free everything" advocates tend to be so much against the freedom of the individual to refer decisions/actions to a collective when that happens to be the better course of action.
“Free market” isn’t “free everything”. In a free market the bid-ask spread is king. It’s just another kind of tyranny, which, like all such things, benefits those who are set up to take advantage of it, and leaves most others in the dust.
Maybe your lack of understanding is because you're arguing against an imaginary straw man?
A group forming a collective and negotiating as such fits perfectly fine with free market ideals. This exists in many forms that all manner of libertarian, free-market, whatever label people are OK with.
When that group uses violence, threats and coercion to their benefit is where people object. Examples - blocking an employer from hiring non-union employees, harming or threatening to harm workers during a strike, forcing people to join a union to get/keep a job, preventing new workers or new firms from entering the market through regulation, etc.
If you want to say "You can hire these 20 union workers at a rate of $$$, we all stand together" that's fine. When you say "You need to hire these 20 union workers, and if you don't, we'll surround your business and threaten, harass and intimidate the people you hire instead" is where you've jumped into violence and extortion rather than free market negotiation.