That's only the case for content purchased through the Kindle store. You're completely free to put content from other sources on the device, and Amazon actually

That's only the case for content purchased through the Kindle store. You're completely free to put content from other sources on the device, and Amazon actually goes to great lengths to make this as simple as possible: Beyond appearing as a USB mass storage device, every Kindle has a unique email address. Send a document to that address and it's instantly delivered over "whispernet" to the device.