Not GP, but agree with them. In my opinion: because you don't show respect for them. What would your reaction be if someone said that to you and you were powerless to do much about it?

Of course, this is all judging just by the tone of your posts, so it might not be (and I hope it isn't) a true reflection of reality. Kids are smart, they just lack experience. But they still deserve respect from day 1, and if they have it then most of the usual parenting problems never surface.