Amazon can, at its pleasure, suspend your account: for example, if you return too many books or other purchases. This also means that your Kindle can no longer get virtual books and subscriptions you already paid for. From The Consumerist:
Your Kindle still works, and the books you already bought for it will work, but you can't download those books ever again (better have made a backup on your PC!), you can't receive your magazine, blog, or newspaper subscriptions on it anymore, you can't email documents to Amazon to have them converted and sent to your Kindle, and you can't buy any new books for the device. That $360 device only works so long as Amazon decides it will work.
What better example of how DRM makes "piracy" mainstream? If it's the only way to stop companies deleting the things we buy, otherwise law-abiding people will be happy to get their hands dirty.