Amazon has faced numerous consequences of that decision, including being hunted down with a lawsuit. In an attempt to make amends, the company has apologized to users. For the affected, Amazon is offering to either replace the deleted digital books with a hard copy, or compensate in the way of a gift certificate or refund. The "buy back" is an opt-in scenario, requiring affected customers to contact Amazon and request reimbursement.
This is a clear-cut example of how companies can use DRM against the consumer. In this instance, the people who purchased books ultimately had no authority over what happened to the content. If you buy a physical copy of a book, it is yours indefinitely -- buy the same book in a digital format and someone up the chain from you could effectively deprive you of access.
Without certain protections in place, DRM systems are prone to abuse -- and it's easy to see why Amazon is taking so much criticism in this particular case. While it is good that Amazon is doing the right thing by offering compensation, you can't really give them kudos for this one. The problem originated several months ago, and they acted without permission or notification to their customers.