This is something I wanted to bring up to make someone think twice before they went to a website for an illegal download of a book in order to get it for free. This was a big issue years ago with music, but what differs between writers and musicians is that writers do not make as much money. If CD sales slow down, recording artists have a multitude of other avenues to earn money: iTunes, concerts, appearances, endorsements, royalties, public performances, rings, etc. You'd be surprised at how many avenues there are to earning money for a recording artist. For a writer, it's book sales. We are not rock stars.
With the increasing popularity of ebooks comes the increase in theft. You wouldn't walk into your bookstore and tuck a book into your purse, so why would you do it on the internet? (Not to mention many of those sites will put viruses on your computer) The problem is, it's not "just one person". Each person adds to the total, and those reduce sales for the author. If one person stops, it makes a difference, because odds are others have also stopped. Why is that such a big deal? Well, I'm not one of those authors who charges $35 per book. Even then, their percentages from that book are slim. I've heard people say they can't afford to buy all the books; but often they are accumulating more books than they'll ever read. I've always saved my money for things I really wanted, added items to my wishlist, and budgeted myself.
Reducing sales reduces funds that are needed by authors for editing services, proofing, marketing, cover art, and other expenses entailed in the process of making a book. You're essentially stealing from the "book fund jar". If the jar becomes low or empty, exactly how is that author going to continue to publish more books? How will you tell that aspiring 9 year old writer when they grow up they'll have people steal their work and won't get paid for it?
I think for indies, it's particularly pocket pinching. It may take us anywhere from 3 months to 2 years to write a book, and they're available for extremely reasonable prices compared to traditionally published books. So when 99 cent or $2.99 books start appearing on free download sites, exactly how much of a pinch in the consumers pocket is that 99 cents for a book that took 2 years to write? You can't even buy a hamburger for that money.
Each time you purchase a book, you are supporting the artist in not only the work they have done, but helping to secure future book releases.
And remember, there are a ton of books you actually can download for free on Amazon, not to mention your local library! Check 'em out. :)