How about any public sector union? They do services that have no competition, allowing them to extract taxpayer money with no substitute providers.
The typical pro-union argument for companies is that they protect workers against abuses by the company.
In the public sector, the union is supposed to protect workers from the citizens? I haven't ever heard of a good theoretical argument for them. Would be interested if someone knew of one.
Public sector unions are fine.
Edit: Your downvotes only make it more obvious that you don't really have a counterpoint to the police's homicidal tendencies nor their unions' participation in crafting legal protections for their actions.