The fact that Secret Service, NCIS, and even DHS personnel were apparently fooled about the authenticity of Taherzadeh and Ali doesn’t actually surprise experts in the field. There’s a human tendency to accept people are who they say they are.
A cynic might say that this makes people an easy mark. Some people will take advantage of the willingness of others to trust them. This is part of the reason spam and scams work: people might not be the most discerning, particularly when the pitch looks viable.
Yet, this giving people the benefit of the doubt increases sociability, cooperation, and trust in the long run. Not everyone will take advantage of you. Humans like to be liked and work with others. Humans are social creatures whose social relationships and networks enable them to do things far beyond what the might accomplish as individuals (read more about this in Connected).
Directly confronting someone about suspicions we may have of them also is a risk. It introduces conflict and an opportunity for both the questioner and the other to be uncomfortable. As we practice impression management, we generally want to avoid losing face. Ignoring a text or email is relatively easy to do; the stakes are higher when the other person is in front of you.
This does not necessarily mean that security agencies should not have structures and/or actors who try to counter these human tendencies. But, the task may be very difficult in light of how humans want to behave.