Let's face it. Many traditional authors may not be rich, but at least they have a publisher to hold their hand. The publisher provides editing, proofreading, cover design, formatting, marketing, etc. Indie authors, on the other hand, do all these things on their own. We know exactly how much everything costs because the money comes from our pockets, and if things go south, we can't just shrug it off like the traditional authors who have nothing to lose because we feel the pain of every failure in our bones, and many indies don't break even on some, or even all, of their books. The services required to make a book or even a short story do not come cheap, so why not get tipped?