agreed, just like attorneys fees and insurance premiums.
For employees, union dues are not "paying for something they already get for free". It's more like insurance premiums or attorney's fees than anything else. The purpose of a collective bargaining agreement is not to punish anyone. CBA's are simply the spelling out of provisions agreed to by both sides and of action to be taken if either side does not abide by the agreement. It benefits both management and employees to have this in place.
Workers enter into CBAs because they have to, because the employers didn’t abide by their agreement, just like hiring a lawyer to sue someone who didn’t abide by their agreement.
If I agree to sell a car to you for $5, and then change the price to $6, I acted in bad faith and you have the right to sue me. If I’m an employer and I act in bad faith, you can sue me individually or you can organize to enforce contractual agreement.
If I offer to sell you a car for $5, and you hand me the $5 and I hand you the keys and title, why would you hire a lawyer?
Why did the players enter into a CBA with the league? Because the players never were allowed free agency or received raises from the early 1900s to the 1960s. The owners acted in bad faith and the union worked as it was supposed to.