In March 2015 Amazon augmented their cloud system that requires users to subscribe to Prime in order to store e-books, photos and video. If you are not a Prime member, it costs $11.99 per year. When this new system went live, it augmented their terms of service no longer allowing companies to use the automatic system to deliver paid content to customers Kindles. Basically, any company that sells e-books in MOBI format can no longer deliver the books directly to Amazon branded e-readers.
Baen will continue to sell e-books in the MOBI format and has now posted detailed instructions on how to manually copy them over to your Kindle via USB.