Not true at all, you can buy the amazon books which usually have the best price, but if you want to buy them somewhere else (ibooks or even kobostore) or get them for free on project gutenberg or any other source all you have to do is to load them on an
app called calibre (available for mac, linux and
windows) which will automatically convert the books into the more appropriate format and send them by email to amazon cloud which will make them appear almost imediatly in your
kindle.