Do they really think that if some ISIS guy flies in, he's going to fill out the customs form honestly and say "Yep, my Twitter handle is @jihad4lyfe!" It's preposterous to think this would be effective at gathering any sort of reasonable intelligence.
The USA citizenship application form has the question: "Have you ever advocated (either indirectly or directly) the overthrow of any government by force or violence?" I remember there used to be something similar on landing cards about overthrowing the US government. There are various apocryphal stories of wits writing "sole purpose of visit".