This is exactly the sort of thinking that led Sony to try and develop UMD as a portable media format.
People buy movies, they don't buy UMDs and DVDs. If someone buys a movie on DVD, it is extremely unlikely they are going to turn around the next day and buy it on UMD, since they already own the movie.
The same, I suspect, is largely true with books. People buy a book, and whether they buy it as a paperback or a hardcover or an audiobook or an eBook, they do not then wake up the next day and decide to buy the same book again. There's no reason to, because they already own the book.
When I need to take a trip, and I'm going to drive, I'll sometimes go out and buy an audiobook to listen to in the car. I've never felt a need to buy the paper version of an audiobook I already own, or vice versa.
The Authors Guild, then, is contending that if you buy the eBook, and use TTS (even perfect awesome TTS with really good intonation and feeling), then they've lost an audio book sale, because if the eBook reader didn't have TTS, you would have bought the audio book.
What the Authors Guild is missing is that NO ONE DOES THIS; if the eBook reader DIDN'T have TTS, and I wanted the audio book, I would have bought the audio book and not the eBook. I would still only buy the book once.
Sure, the TTS is a nice feature, and of course it makes the eBook more versatile, but it hasn't lost an audiobook sale, it has replaced the audiobook sale. If the TTS wasn't there, I still wouldn't buy the book twice, so there's no sense giving them double royalties for eBooks.