In a truly free market, the Union could offer the services of it's members to the employer, but the employer would not be FORCED to buy labor ONLY from the Union
I'm not sure of any law to which you may be referring that requires an employee to get labor ONLY from a union. I may well be wrong, and if that's so, then that's so. The situation to which I think you are referring is a "closed shop," in which an employee must join the union in order to work at a particular job. Whether a job is "open shop," where membership is an option, or "closed shop," where membership is required, is totally the choice of the union; it is especially discussed when the workers initally vote to form a union.
I said all that to say, in that situation, I have no problem. Big Government is not requiring anything; rather, the union has chosen, for (presumably) the benefit of the workers, to require membership. The worker knows the situation going in and accepts it. There are other jobs if he chooses to not want to take part in collective baragining.