Writers can do that.

There's no requirement for a writer to submit content to Apple (iTunes store) or Amazon (Kindle store). A writer can format documents in .mobi, .epub and .pdf files and sell them through any digital storefront (either home brewed or something like leanpub among many others)

If a writer wants the exposure from being on the Kindle Store or iTunes Store, that's another matter. But there's no requirement that a writer sign up for such a distributor.