Nonetheless, this has been done in many places and it led to ridiculous results. For instance, if you have a booth at a convention center in Las Vegas somewhere, just try carrying your own box from your car to your booth. Just try! You will immediately get screamed at that it's a union job.
Sure, but you can't attribute that to unions and say that's a universal negative effect of unions, the same way that you can't say knives are universally bad because they've been used to stab people in the past.
This is an example of how it specifically is. The deal/law with the city/state/municipality/whatever is that they can't hire someone who's not associated with the union. That results in ridiculous shenanigans I just described.
It's more a function of a state or municipality's negotiation with a particular union, and can be a rational decision for a municipality to make. The choice is between hiring only union people, or not being able to hire any union people.
You could say that for ISP laws as well. "It's more a function of a state or municipality's negotiation with a ISP, and can be a rational decision for a municipality to make. The choice is between having an ISP/good broadband or not having one at all."
The point is that the mere existence of a union is not a sufficient condition for union-only laws; unions exist in places where such laws don't exist. As an argument against unions this is a slippery slope fallacy.