Actually, you can return ebooks early, but you can't do it through your phone. Download the ebook to your computer and use ADE to open/read it. Once the book is in your library you can choose the option to return it.
It's inconvenient, but it works. I've done it a few times--especially when an ebook that I've been waiting for comes available and I need to ditch one I've already got to check the new one out.