The government argues that searches at international borders is a "reasonable" search, thus the 4th amendment's prohibition on "unreasonable searches and seizures" doesn't apply.
The EFF disagrees. You can read their reasoning in the linked article.
The EFF disagrees. You can read their reasoning in the linked article.
It's a way government can eschew all constitutional protection for private data, by simply waiting for the target to cross a border, practical for a majority of citizens. Data is substantially different from physical possessions because it's very portable and most people do indeed carry vast amounts with them when traveling.
Furthermore, in the modern world data import or export offenses (espionage) do not employ physical border crossings, so there is almost no reasonable offense the search could uncover.