The difference is.... the moment you tell somebody your $100 bill is counterfeit it loses all value.
And this would be true in a country that didn't have copyrights. Therefore, it isn't right to equate copying a bill with copying a book.
Yet telling someone that you have made a copy of a song or a book doesn't lose all value. People can still listen to the copy of the song (in exact digital detail), and they can still read the copy of the book. In other words... they are getting the full value of the product from you even though it cost you nothing to produce and you have little to no investment in it's creation.
It's not true that it costs nothing. Even time to make a copy costs something. People will know this and pay accordingly. Just note copying a bill and copying a book cannot be equated. It's the same with a signature; copying a book and copying a signature are not the same for many of the same reasons.
One of my sources of income is digital clip art. Myself, My brother, and My father (God rest his soul), invested years of our lives drawing the images and converting them to digital clip art files to sell on the internet.
I have found people that take my files and resell them on eBay and Craigslist. This is obviously devastating to my income because it robs me of potential sales. You would side with the thief who is stealing from me and my family. That kinda offends me.
But I don't side with the person violating copyright. If the law of the land is copyright, we must abide by that law for the reasons I gave earlier in this thread.
In a country with copyrights and patents, the market is skewed because the government creates false signals. This is what you are blaming me for, and I'm not the one doing it.
If the country did not have copyrights and patents, then you would have a clear understanding of what people wanted and what they would pay for it. You might have other delivery systems, or other addons to your product. There is no way to know because we don't live in that world.
This is exactly the same as our criminal justice system. It's really messed up, and we advocate for the correct system, but we live in and abide by the current one. Take any current case and imagine what it would be like in a good justice system; but wait, would we have even had that case if the justice system had been correct? There is no way to know. In the end, we find out what is correct and advocate for that.