Freedom of association.
If workers want to associate together - that is their right.
They also own their own labor - that is THEIR PRIVATE PROPERTY.
It is very much a two way street in the private sector.
You are VERY confused about freedom of association. VERY CONFUSED. Yes, workers may associate with other workers all they want.
THE PERSON WHO OWNS THE WORKSPACE ALSO HAS FREEDOM OF ASSOCIATION, AND MAY FIRE WORKERS WHO DISPLEASE HIM BY FORMING A UNION TO WORK AGAINST HIS INTEREST.
Some private-sector unions are bolstered by taxpayer when government contracts have to be awarded to union workers (outside of the traditional teachers/cops/firemen unions); in the end, the “private sector union” ends up as a quasi-public employee union, and donates and votes accordingly.