Compare the scale of the harm that you see to the direct benefits: these huge unions getting each and every member a solid benefits package, and better pay rates, overtime or limited hours. Even non-union jobs pay better in areas with high union membership. Its one of the only tools with broad benefits to working-poor and lower-class at the cost of the non-working rich.
I think it's an unfair argument to say that unions need further regulation. Regulating them is how we've lost millions of union jobs to contractors, and where we keep seeing large strikes broken up by governments. Regulations often benefit large players over small ones. We'd rather see a plethora of smaller, niche unions, than just one massive union. Competition breeds capability.