The US blocked Daniel Stenberg, free software developer and author of curl, from flying to the US. He was given no explanation, of course.

Occasionally there is a valid reason to deny someone entry to a country. But there is no excuse for being mysterious about why.

That the US so often approves a visit in advance, then turns the visitor away at the time of the flight, suggests that the US is doing the initial checks carelessly. Whatever checks they want to make, if they were competent they would make those checks when the visitor first applies, not later.

The last-minute denial inflicts a big loss, often a thousand dollars or more, for the price of nonrefundable airline tickets. It teaches them that the US government cares as much about them as about an ant.