Adult humans know it is logical to tell white lies in some situations. Every one of us does that. See what happened in the movie "Star Trek Into Darkness" when Spock refused to lie and wrote truthfully in his report how Kirk broke the Prime Directive. Kirk did it to save Spock's life. For saving his friend's life, Kirk lost his ship and job as Captain of the Enterprise because his Vulcan friend cannot lie.
Kirk: Where I come from, if someone saves your life, you don't stab him in the back.
Spock: Vulcans can not lie.
Why can't Vulcans lie when it is a logical thing to do sometimes? Why can't Vulcans, with all their powers of logic, cannot see the logic?