"Even though the Buddha was generally open to cross-questioning from his listeners, the fact that he was offering his teaching as a gift meant that he held the right to maintain firm control over what he would and wouldn't give. This meant exercising control over two things: the questions he would and wouldn't answer, and the questioners he would and wouldn't respond to."
From: Skill in Questions: How the Buddha Taught
by Thanissaro Bhikkhu (404 page pdf)
With metta / Antony.