Earlier quoted context omitted.
By that definition, companies are workplace monopolies. So are you against companies ? If you want to argue you can choose a different company, by the same token you can choose a different union by choosing a different company.
I believe what GP was alluding to is that if a workplace is unionized, the worker does not have a choice of wanting to be a part of the union or not(in a non right to work state), and so essentially a monopoly. IF the worker wants to work for that employer, he can only do so via the union. No way for the worker to work for the employer and not wanting to do absolutely nothing with the union.
Once an employer can no longer hire non-unionized employees, it has to either surrender to the union requirements or close altogether. This is a monopoly in its best.