What can you do with a Kindle except buying books from Amazon? As far as I know, you can read books without DRM and you can surf the web. That’s equally true for all iOS devices. What else does a Kindle allow you to do that you cannot do on an iOS device sans apps?
It seems to me that the difference is what you purchased the device for. Based on advertisements, I'd say the usual goal of an iOS purchase is to run apps and the usual goal of a Kindle purchase is to read books. You can read books on the kindle that amazon doesn't approve of, but you can't (reasonably) run apps on an iOS device that Apple doesn't approve of.
You'd be hard-pressed to call the Kindle a general-purpose computing device; its hardware limitations make it more or less useless for anything but reading. Apple's limitations on what you can do with your devices are artificial and arbitrary.
What can you do with a Kindle except buying books from Amazon? As far as I know, you can read books without DRM and you can surf the web. That’s equally true for all iOS devices. What else does a Kindle allow you to do that you cannot do on an iOS device sans apps?