The point of that decision by a right-wing court was to make it harder for public sector workers to stand together in unions. Unions are democratic institutions, with dues and leadership decided by the membership, and the idea that some people pay and some don't even though the entire unit is represented doesn't make a ton of sense. Just like it wouldn't make sense if 2 people in a unit of 1000 wanted to be in a particular union to just say those 2 can bargain collectively.
The weird "speech" argument was basically that their worksite issues are inherently political. I disagree. Unions have separate political funds from the worksite stuff that are optional.