How are free ebooks piracy?

No, you're not stupid. You are seeking knowledge, which marks you as intelligent! The darkside of downloading books is that IF the book is still under copyright, you are cheating the author out of his/her royalties. The exception is, if the author has placed the book on free download, no problem. But there are zillions of good books that are NOT in copywright, and you are welcome to all of them. Take a look at Gutenber.org to see a vast collection of books that are out of copyright and free to download.