[derefr]

The carbon emissions would still be there, but they might not be nearly as toxic/hazardous. (See my reply to a sibling comment.)

On the other hand, they'd be happening over land, where people live; instead of over water, "merely" killing marine life, so that might be a wash in policy-makers' minds.

To be clear, though, I'm not arguing against using cargo ships for domestic logistic as a concept; just the current implementation. Cargo ships that didn't use bunker fuel would be an unalloyed ecological win compared to both domestic ground logistics, and the current implementation of domestic marine logistics.