I would guess part of the reason is because they aren't rational - they are upset that things changed and interfered with their snooping even though it has been irrelevant since radio and code books have existed. If I say "Aunt Bertha had a baby." without context you have no idea if I am being literal or signaling something sinister.

A far harsher and less charitable theory is that border control is where paranoids, xenophobes, and authoritarians accumulate and the point is control and punishing "the other" instead of actual effectiveness.