If you have an apple and I have an apple and we exchange these apples then you and I will still each have one apple.  But if you have an idea and I have an idea and we exchange these ideas, then each of us will have two ideas.

So if you have an apple and I have an apple and I steal your apple, I will have two apples and you will have no apples.  Sucks for you.
But if you have an idea and I have an idea and I steal your idea, then I will have two ideas and you will still have one idea, so really, you aren't any worse off than before.

I wonder if I can use that for a justification of online piracy...