I think the issue is that a non-Jones ship going from DR to PR could then not continue on to another US port, which means that PR would have to have enough demand to unload the entire ship there or at another foreign port.
The issue really comes into play with the fact that a non-Jones ship couldn't stop at a US port on the way to another US port, and that really hurts smaller outlying ports that are between a foreign nation and mainland US.
It's a bit of a further tangent, but my understanding is that this has a particular weird impact on cruise ship destinations and related tourism cash/investments. Because cruise ships aren't exempt from the Jones Act either and are often foreign flagged (many cruise ship companies are nominally headquartered in the Bahamas and fly Bahamanian flags on their ships), they also have to make sure that they alternate ports in the way that meets the Jones Act. They can't go from Florida or New Orleans straight to Puerto Rico without stopping somewhere else first (and with US sanctions still "weird" with Cuba the obvious stop isn't generally possible either) or travel directly between Puerto Rico and the USVI without a stop somewhere in between (such as DR).
This seems silly for a number of reasons. I know a bit too much about it because a cruise charter group I go on cruises with has wished for certain itineraries to make logistics easier (PR has better concert logistics than most other islands, for instance, if you are trying to set up a single day concert in a park), but there are too few available PR routes for the sake of variety precisely because some of those routes that might be desirable are basically illegal under the Jones Act. It's not a showstopper in this group's case, but it is silly.
A non-US ship can go to PR and then another US port. From the article:
"The concrete results are that a foreign ship can enter a U.S. port to drop off its foreign cargo, it can even from there sail to different U.S. ports to drop off other foreign goods, but it is not allowed to pick up any domestic goods in a U.S. port and bring those goods to another U.S. destination if it doesn’t meet the four conditions listed above."
Thanks for the clarification. I was running off of memory from a Peter Schiff podcast where he talked about the Jones Act.
That is still an issue though. Those ships need to try and maximize on board cargo at every port. They have wasted dollars if they don't. There is zero reason for this law to exist except to prohibit free trade and stomp out domestic competition.
The issue really comes into play with the fact that a non-Jones ship couldn't stop at a US port on the way to another US port, and that really hurts smaller outlying ports that are between a foreign nation and mainland US.