By preventing large numbers of people from working at all.

Joining unions is impossible for many. I once lost a job because the union insisted that I be let go rather than let me join. It was not a performance issue.

What if someone is perfectly happy to drive for $1 less than the union negotiated? Why would you use the force of government, police with guns, to keep them from selling their labor a dollar cheaper. Especially when they’re perfectly happy to do so?