What is a union?
A Union is an employee association of workers, formed to bargain with their employers, in regard to working conditions and wages.
The problem on this issue is that any discussion of unions becomes obfuscated by laws, regulations, and mis-specified rhetoric.
If an employer can refuse to give work to an employee, why can’t employees refuse to give work to an employer?
Take coercion, violence, the strong arm of the government, and everything else out of the discussion, and there should be no reason they can’t.