[indigo945]

That's actually precisely the reason. [1] It's not always easily possible to arrest somebody for having been or even being a terrorist abroad, but you can definitely arrest them for lying on their visa application papers.

[1]: Well, probably. The Department of Homeland Security doesn't give an official reason: "The Department of Homeland Security did not explain the reason for this question, and it is not clear why any terrorist, spy, saboteur or mass murderer would answer “yes.” Doing so, whether or not by mistake, does not mean that the person will be banned from the United States, but it doesn’t help." (https://www.nytimes.com/2018/11/30/world/europe/terrorist-qu...)

[seanmcdirmid]

For the same reason that it is easier to arrest drug dealers, pimps, mafia dons, etc… on tax evasion charges than the more immediate crimes they committed.