Unionization is well beyond freedom of association. It's freedom to enforce a monopoly. Can you be consistent, having no objections if others do that too? For example:

Airlines form their own sort of union. (we might call it a cartel) They decide to agree on prices instead of competing. Prices double.

Fueling stations do likewise. They agree that gasoline should be $12 per gallon.

Cell phone providers decide that you should pay $1 per megabyte. They all agree, being in their union, that they won't work for less.

Good?