It's way easier to understand when they just want to attack labor and absolve capital, it really is that simple for 99% of the discourse you read online.

The protections offered to unions goes far beyond standard freedom of association. Generally I think that is okay, but not in the case of public unions.

I think you may be misunderstanding. The right to join a union is the right to association. Right to association is not something provided by a union. In other words, you have a right to collectively bargain for certain working conditions and that is guaranteed by the 1st amendment.