Because that's what a large percentage of their parents tell them
And God forbid parents be allowed to raise their children according to their own values. We certainly can't have that, not when we can use the power of the state to forcibly indoctrinate them!
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SmartMuffin - Because anything less would be uncivilized - http://img.imgcake.com/smartmuffin/barkleyjpgde.jpg
http://dudewheresmyfreedom.com/