I think a device's ability to read text is nothing like an audio book.
Text read by a computer still has errors, doesn't sound as good as a real person, and can't be applied to books with images, mathemathical formula's, so is restricted in many ways.
I would say there's no valid ground to disable the speech function off kindles, as you could download an ebook at home, and use a L&H speech decoder, or whatever text to speach decoder for free, and listen to the book that way.
I do think that decoding text requires more cpu and battery life. The battery life is pretty limited on ebook readers for functions like that and listening to mp3's.
Aw well, I hope Bebook or other far away Asian manufacturer will get such functions in their ebooks and on the market (ebay for instance), so we could have it.
It seems those asian markets don't really bother with European and US copyrights/licences.
I read Internet instead of books.