The Department of Homeland Security (DHS) has refused to waive the Jones Act, a relatively ancient law signed in 1920 that restricts shipping between American ports (domestic trade) to predominately American-made ships and crews, to help Puerto Rico recover from Hurricane Maria.
The law was waived to aid recovery efforts in Florida and Texas following Hurricanes Harvey and Irma, but it has not been waived for Puerto Rico.
According to Trump himself, the law hasn't been waived because the American shipping industry wouldn't like it.
“On Puerto Rico, Mr. President, why not lift the Jones Act like you did in Texas and Florida?” a reporter asked as Trump made his way to Marine One.
“Well, we’re thinking about that,” he responded. “But we have a lot of shippers, and a lot of people — a lot of people that work in the shipping industry that don’t want the Jones Act lifted. And we have a lot of ships out there right now.”
In other words, Trump has placed the profits of the shipping industry ahead of human suffering.
That's hardly surprising, but there it is.