They did this out of the kindness of their hearts ... well, and because the Apple Book Store was offering that rate. Nothing like a little competition from somebody who makes you nervous to stir things up.
Notice that the prices of Kindles and Nooks have also fallen in response to the best-selling iPad, which has already blown past both dedicated readers in terms of sales, by the by.
What this means is that my eBooks on Amazon.com won't be available for 2-3 days as they rejigger the amended files. (You have a choice when you go look at your shelf -- and why anybody would choose half as much royalty is beyond me, but you do have to opt in to get the higher percentage.)
Onward and upward ...