Not one device that I know of (at least the ones that I have) prevents you from sideloading your own ebooks. iPad initially made it difficult but then updated iTunes and now it's easy. Kindle never prevented sideloading; it even provided more than one way to get your ebook on the Kindle (USB, email to the Kindle, download from the browser). From what I've read, Kindle Fire will not only allow sideloading of books and other content (music, movies), you will also be able to sideload Android apps without rooting:
"
Yes, you can sideload Android apps using a USB cable and mass storage."
Also, "
Amazon isn't doing anything special to prevent techies from "rooting" and rewriting the software on its powerful yet inexpensive new tablet, Jon Jenkins, director of Amazon's Silk browser project said."
If Amazon is not preventing rooting,
a fortiori, they won't prevent sideloading of content.