Unions strengthen workers. A company that negotiated 1000 small contracts has the upper hand in each of them, if it is forced to negotiate 1 big contract, then they are on equal footing.

So if only the people, the workers produce stuff, and the form of organization like the union is parasitic then the cooperation does not produce stuff as well.

The only way to have any negotiating power is to present yourself as a single, unified entity. A union. Single workers or small groups have no power against a corporation. Just like individuals have no power against a government. It's only when they come together that they can effect change.

So fighting against unions is the same as fighting against people's right to associate since any group can have the same shortcomings unions do.