"Upon your payment of the applicable fees set by Amazon, Amazon grants you the non-exclusive right to keep a permanent copy of the applicable Digital Content..."
Sounds like you buy it, you have the right to keep it.
Except it also says, "Amazon reserves the right to modify, suspend, or discontinue the Service at any time, and Amazon will not be liable to you should it exercise such right."
So... Amazon perhaps claims the right to go into any Kindle at any time, and remove all the ebooks therein.
Nowhere in the TOS does it say "if Amazon decides that it did not wish to provide a particular bit of digital content, it has the right to remove it from people who purchased it in good faith." It says both, "you bought it; you get to keep it" and "Amazon can revoke all your access at any time, with no warning or explanation" (which should properly terrify every Amazon customer; who knows when they'll decide to "remove all anti-patriotic content?"), but it never implies that Amazon will be removing individual purchases.