> But why is this bad policy?
That's not the right question to ask. The right question to ask is 'Who is this bad for?'
The Jones act, for instance, is good for US shipworkers, and for enforcement of US shipping laws, but moderately bad for Hawai'i, and really, really bad for Puerto Rico.
Who do you care more about? Preserving the comfort of the continental American middle class employed in maritime transportation, or a bunch of people in Puerto Rico? The answer to that question determines how you see the Jones act.