[murphyslaw]

I've also noted that issue. I know of several people at my former job who traveled to the San Jose office and were turned back for some inane reason that had never been an issue before that time. US Border Guards have extreme latitude and they seem to like their power a bit too much.

Problem is that a single rejection forces you to go through the Visa process instead of the Visa-free process for the rest of your life: one of the questions on the entry form is (or was) "have you ever been denied entry", next to "are you a nazi war criminal" (not a joke). And answering "yes" on the former guarantees a whole lot of pain. If you answer "no" you're committing a felony.

The USA's entry system is inane, utterly dependent on the person in front of you and downright scary to me.

I've known conferences to move outside the US for this kind of reason, the pain was just too much and too many people couldn't get in.

[Khaine]

It also occurs if you visit countries the US doesn't like such as Cuba or Iran.

Visited Iran when sanctions weren't in place, no ESTA for you. You get treated like a criminal, and have to pay more money for a proper visa.