Why do you give away your books? Giving away ebooks gives me artistic, moral and commercial satisfaction. The commercial question is the one that comes up most often: how can you give away free ebooks and still make money?
For me for pretty much every writer the big problem isn't piracy, it's obscurity (thanks to Tim O'Reilly for this great aphorism). Of all the people who failed to buy this book today, the majority did so because they never heard of it, not because someone gave them a free copy. Mega-hit best-sellers in science fiction sell half a million copies in a world where 175,000 attend the San Diego Comic Con alone, you've got to figure that most of the people who "like science fiction" (and related geeky stuff like comics, games, Linux, and so on) just don't really buy books. I'm more interested in getting more of that wider audience into the tent than making sure that everyone who's in the tent bought a ticket to be there.
Ebooks are verbs, not nouns. You copy them, it's in their nature. And many of those copies have a destination, a person they're intended for, a hand-wrought transfer from one person to another, embodying a personal recommendation between two people who trust each other enough to share bits. That's the kind of thing that authors (should) dream of, the proverbial sealing of the deal. By making my books available for free pass-along, I make it easy for people who love them to help other people love them.
What's more, I don't see ebooks as substitute for paper books for most people. It's not that the screens aren't good enough, either: if you're anything like me, you already spend every hour you can get in front of the screen, reading text. But the more computer-literate you are, the less likely you are to be reading long-form works on those screens that's because computer-literate people do more things with their computers. We run IM and email and we use the browser in a million diverse ways. We have games running in the background, and endless opportunities to tinker with our music libraries. The more you do with your computer, the more likely it is that you'll be interrupted after five to seven minutes to do something else. That makes the computer extremely poorly suited to reading long-form works off of, unless you have the iron self-discipline of a monk.
The good news (for writers) is that this means that ebooks on computers are more likely to be an enticement to buy the printed book (which is, after all, cheap, easily had, and easy to use) than a substitute for it. You can probably read just enough of the book off the screen to realize you want to be reading it on paper.
So ebooks sell print books. Every writer I've heard of who's tried giving away ebooks to promote paper books has come back to do it again. That's the commercial case for doing free ebooks.
Now, onto the artistic case. It's the twenty-first century. Copying stuff is never, ever going to get any harder than it is today (or if it does, it'll be because civilization has collapsed, at which point we'll have other problems). Hard drives aren't going to get bulkier, more expensive, or less capacious. Networks won't get slower or harder to access. If you're not making art with the intention of having it copied, you're not really making art for the twenty-first century. There's something charming about making work you don't want to be copied, in the same way that it's nice to go to a Pioneer Village and see the olde-timey blacksmith shoeing a horse at his traditional forge. But it's hardly, you know, contemporary. I'm a science fiction writer. It's my job to write about the future (on a good day) or at least the present. Art that's not supposed to be copied is from the past.
Finally, let's look at the moral case. Copying stuff is natural. It's how we learn (copying our parents and the people around us). My first story, written when I was six, was an excited re-telling of Star Wars, which I'd just seen in the theater. Now that the Internet the world's most efficient copying machine is pretty much everywhere, our copying instinct is just going to play out more and more. There's no way I can stop my readers, and if I tried, I'd be a hypocrite: when I was 17, I was making mix-tapes, photocopying stories, and generally copying in every way I could imagine. If the Internet had been around then, I'd have been using it to copy as much as I possibly could.
There's no way to stop it, and the people who try end up doing more harm than piracy ever did. The record industry's ridiculous holy war against file-sharers (more than 20,000 music fans sued and counting!) exemplifies the absurdity of trying to get the food-coloring out of the swimming pool. If the choice is between allowing copying or being a frothing bully lashing out at anything he can reach, I choose the former.