As far as I know it is not a new policy. We were not aware that it affected our use of the Send to Kindle feature until they contacted us referring to their
Terms of Use and indicated that it was not allowed. I was very surprised that it affected our use as O'Reilly technical books has a similar feature that ships books to your Kindle on purchase.
We were very disappointed to have to pull this feature as, for many users, sideloading is too technically involved. I know I would never be able to teach my grandmother how to sideload a book to the Kindle Fire that I bought her for Christmas.