Please correct me if I am wrong, but doesn't this law simply prevent a situation where a non-union employee would be forced to pay the union fees even if he doesn't want to be a part of the union in a unionized workplace? If yes, how is it a bad thing for employees?

Genuine question, because I don't understand the issue. If they want to join a union and pay the associated fees, they are welcome to. If they just wanna work a job without joining a union and paying their fees, they can do that as well. How is it a bad thing, unless you consider forcing those employees who don't care about unions to still contribute fees a good thing?