Unions did not get into the workplace by a voluntary agreement with businesses but forced their way in by coercion. And they maintain their position by coercion. What happens if the business decides to part ways with the union? Do they take their workers and leave peacefully?
No one argues that if you don't want black lung disease you don't have to work in a coal mine, so why argue that if you don't want your money to be taken away against your will and spent on politics you disagree with, you don't have to accept a job in a union shop? In both cases they are economic decisions - not made of free will. Libertarian philosophy is the belief in individual rights and those rights should never be subordinate to the rights of the collective.