The problem is that anything a union does for the employees makes the lives of all employees better. If you make payments to the union optional, you'll get the usual free-rider problem, in that people will eventually figure out that if they stop paying money to the union, they'll still reap the benefits anyway, and the union will likely be woefully underfunded.
Of course, for those who oppose even the existence of unions themselves, this is not a problem, but creating a union and then making the payments to it by the employees entirely optional is just bad planning.
This fits my views exactly.
Everyone benefits from the existence of unions, regardless of whether they are in a union or not.
In that sense dues are similar to taxes. Even if you don't like the government and think we would be better off without it, you still have to pay taxes, because you still gain benefit from the government in the form of defense, schools, roads, etc.
Should taxes be optional?