For me, it's because I set my brother up for the blame. I was a manipulative little demon. When I was really little (my mom tells me this story) my brother and I would be in the backseat and suddenly I would burst out sobbing because he hit me. Unprovoked. Just “whap!” And cute little innocent me would bawl my eyes out and getting him in big trouble. He'd say that I started it, but why would a cute little toddler do something like that? Clearly the 5 year old is to blame. But then my dad put down his rear view mirror and watched me poke my bother relentlessly until he'd have enough and hit me to get me to stop. Then the waterworks.

But sneakiness aside, it's because they should know the rules and should know what is appropriate and what isn't.