It's a bit of a joke and hack, I think. There are better options for visually impaired people, including android or apple tablets that will do both audio books and TTS, with a better battery life to boot.
I strongly suspect this is in response to some nuisance lawsuits that have prevented the institutional contract sales of Kindle devices in the past, due to lack of accessibility options. Rather than the clients agreeing to supplementing Kindle roll out with accessible devices for those who need them, Amazon can provide a inexpensive accessibility add-on.
(Like this one, from a few years back:
http://arstechnica.com/tech-policy/2...aired-settled/)