That's a symptom of the same problem of union weakness. If it were easy to unionize and hard to bust we'd have higher union density, like we used to have and unions would be strongly representing their members, like they used to. Or else members would rip the leadership, or form another union. When forming a union is hard and busting is easy (various curbs on right to strike being busting strategy), people try to hold onto the little leverage they get from their less than effective, employer-compliant union because it's often better than having no union, instead of challenging the union leadership internally to do its job.