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.
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.
"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."
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.