Because then the press would immediately ask questions that force the officials to choose between sending themselves to jail and divulging information that would get people killed and destroy foreign affairs.
They can refuse to answer any question, whether under oath or not. This would just mean that the whole "who cares if I blatantly lie to the populace, it's legal for me to do so" attitude would be gone.
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SmartMuffin - Because anything less would be uncivilized
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