If an ebook is DRM free then in the US (the only place where the Kindle is sold) read-aloud is a protected "fair use" format shift of the material. I don't see how DRM can come into play, since Amazon is certainly allowed to circumvent its own DRM. Publishers may be able to argue that their contract with Amazon does not allow read aloud, but presumably Amazon's lawyers have an alternative theory. In any case, publishers may be cutting their own throats - because if all ebook versions prohibit read-aloud then the Library of Congress allows anyone to circumvent DRM for personal use (see
DMCA Anti-circumvention exemptions).