Carrie, any of the ebook sellers can block access to your account and all of the ereaders with WiFi can remotly remove books from your ereader. It is a hazard of the technology.
Amazon has closed accounts because they have been hacked, a buyer abuses the return policy, or a buyer violates the terms of service by opening a new account when an old one has been closed or by doing something like buying georestricted material.
Once every few months someone will post about the injustice of it all. People get all pissed off at Amazon. Others ask questions and by the end of day 2 it turns out that the Kindle was broken, the "victim" admits that they violated the TOS in some egrigious manner, or the "victim" owns up to returning a ton of goods or ebooks in a short period of time. I would guess that BN has shut down a few accounts but they are not as large as Amazon so we have not heard about it. I know that other companies have removed content from peoples accounts. All of the smaller ebookstores did so when Agency Pricing came into play. Since they did not have contracts with the Publishers, they no longer had the rights to sell the books, and they removed them from peoples accounts. If the books had not been downloaded to someones computer, then they lost the book.
My point? You face the same risk with a Sony or a Nook or a Kobo. They all have WiFi and they can all pull books. You face the same risk at any ebook store. The risk is relativly small with all of them, Amazon sells how many ebooks across the globe and we hear about this how frequently? The only people I have ever seen post on it on this board are people are new to the site or the story is told by someone who does not own a Kindle and found an obscure blog on the matter.
Back your books up on your computer and you will be fine. The folks who bought Ninteen Eighty Four and had it backed up on their computer were able to manually reload the book and had no problems keeping it on their Kindles. Take it one step further and backup your books and remove the DRM.