This was back in the 1990s, when asking for your password wasn't a thing I've ever heard of them doing, but also when me bringing 128-bit encryption software (aka the US version of Netscape Navigator) to Japan, where I was a foreign student, was a serious crime akin to arms smuggling.
Of course, I never found out for sure why I was on that list, and eventually I apparently wasn't on it anymore. But during that time I did read a fascinating article in some magazine, by Nathan Myhrvold (the now-infamous patent troll scumbag). Apparently, he was on the same list, despite being a super rich fat white guy from Microsoft.
His theory was that he got on the list by buying one-way plane tickets in cash. That resonated with me, because I often did the same thing back then… I just never really knew when I wanted to come back, and I didn't have much credit on my one credit card.
That's obviously both circumstantial and anecdotal, however I don't think it really takes too much to get on one of these secret (and very probably unlawful, but effectively un-challengeable) lists.