The true problem with piracy is actually not the issues that others have mentioned with readers disallowing sideloaded files.

The main issue with piracy is that both Amazon and Apple have far more advanced typesetting systems in the custom file types (Kindle Format 8 and iBook format) that you get when you buy from their store.

The .epub format its still based on what, XML? .epub makes up most of what you see when you pirate. If you look closely you can see that most epubs actually are really badly typeset compared to the versions of the store.