I don't understand, how could you morally make the case for someone not to give away free ebooks if it indeed makes them more money? Even if you didn't want the extra money could you not donate it to charity for instance? (I really think it needs to be studied further before we can say exactly what kind of effect giving away free ebooks has on an author's sales.)
You are lumping two very different things in together: giving away free books for whatever reason, promotional or otherwise, and someone taking them without permission.
Most authors do give away free books for promotional purposes where they think it will do them the most good, not where some random somebody gets a pirated copy and tells a friend, "Look I got this book for free. It was pretty good. You can get it for free, too." This is not encouraging other people to buy the book!
But we send out copies to reviewers, or as prizes in promotional giveaways. Sometimes we just give them away to people we think will enjoy them, or to libraries in our communities because we want to support them. There are dozens of good reasons for giving away free books. But we like to decide when and where. We like to decide what is likely to be for our own good; we like to decide when we simply want to be generous.
If a fan writes to me and says they enjoyed a specific book and if I have extra copies of the sequel (I don't for some books), often I'll just send them a copy, and pay the postage myself. They're already a fan, they'll almost certainly buy more of my books, so I'm probably not going to see any financial benefit from giving the book away. Maybe it's just a whim. Maybe I feel it's good karma. Maybe I'm grateful to them for liking my books. But it's my decision and I think other writers should be allowed to make these decisions for themselves. If they think that giving away books will help them sell more books, fine. But before they do it, they would be wise to figure out whether it really does help them sell more books.
If there are to be studies, they should be conducted by unprejudiced sources, not by the pirates themselves. And these decisions should not be based on the fact that writer X says they are selling YYYYYY number of books since they started allowing free downloads to the world at large. This may come as a big surprise to you, but sometimes writers inflate their sales numbers, to make the book look more desirable. Besides, even if they are telling the truth, it may not be a typical result. There may be other factors at work.
But when it comes down to it, it's all about the writer's choice, and those who would take it away. Which is probably a greater theft than that of the book.