It could make sense for foreign visitors (to check for intended visa violations, etc.)

But there is no reason to search the devices of U.S. citizens and lawful permanent residents, who are already authorized to live and work in the U.S. These people would have no reason to break immigration law. And if the government is searching digital content for evidence of other unlawful activity, they should have to get a warrant.

Keep in mind that any digital content you can bring through a border port-of-entry, you can also just send it via the Internet.