Tivoization refers to hardware manufacturers 'circumventing' free-software licenses by only allowing their devices to run versions of the software signed by the manufacturer itself. This makes some people mad because they believe that a free software license somehow implies an unalienable human right to run the software on any hardware of their choosing. Others believe that supplying the source code is enough to satisfy the software license.
Amazon provides
all their Kindle source, and you are free to examine and modify it as you please, and even run it on your own hardware -- provided it isn't a Kindle, because the Kindle (without jailbreaking) will only accept Amazon-signed versions of its software to be installed and run.
Speaking only for myself, I'm fine with people being able to buy a second-hand Kindle on eBay without worrying that the browser has been modified to add a keylogger; others may disagree. The manufacturer has every right to protect its users (and itself) from wrongful or malicious use of the hardware, especially to avoid circumstances such as those described by Anthem above, or worse. But especially in these cases, where the provider and the device are inextricably linked by branding, allowing modified software to run could be a legal/publicity/financial nightmare -- user buys a hacked Kindle, runs to CNN yelling 'Amazon let Russian hackers steal my identity,' pitchforks outside Amazon HQ.