I don't think this is useful analogy. If Kroger raises prices for milk, you can cross the street and buy your milk from Walmart or Aldi or Costco or any of the other hundreds of places that sell groceries.

If the ILWU goes on a slowdown strike, the port authority cannot simply fire them all and hire replacements. Firstly, that is outright illegal in America and secondly, there is no competition anyway. The ILWU has a monopoly on the supply of longshoremen. If all the grocers were entered into a cartel agreement to not sell milk below a certain price, then you'd have a point. There's no evidence that this is the case.

But all longshoremen are entered into an agreement not to sell labor below a given price. This agreement is their union membership with the ILWU.