That relates to sections 11 and 12 of the Customs Act. Basically, you have to answer truthfully all the questions asked of you. The dynamic you would create by not voluntarily responding to those questions that are asked in that few minutes that a border services officer has to figure out who you are and what kind of risk you may pose would probably trigger a process in his mind that would result in your needing to go to a secondary line, so that someone else can spend a little more time with you.
On May 6th, 2008. See this statement in context.