Apple isn't the only company with a proprietary device that they want total sale control over. Amazon's Kindle, for instance, is also designed to make use of online shopping exclusively through sanctioned Amazon avenues. Thus, when the retailer learned about a third party method used to extract the PID information from a Kindle, with the end goal being content delivery from a non-Amazon source, they acted quickly.
MobileRead published instructions on how to use a small piece of software to get your Kindle's PID, and ended up receiving a DMCA cease and desist notice from Amazon, telling them to immediately pull the software and the instructions on how to use it. Though the site doesn't necessarily admit any wrongdoing, they complied with Amazon - but not before many mirror sites began replicating the content.
Their goal was simple: allow people to buy eBooks for the Kindle from anywhere. Given how much Amazon has invested into the device, you can't really blame them for waving the DMCA at anyone who would try to circumvent their direct channel to it. Many other companies, such as Apple, do exactly the same - despite protests from users who think the choice should lie with whoever owns the device. Amazon is resolute, though, claiming that information like the Kindle's PID is protected by the DMCA and users have no right to extract it.
This instance is yet another in a series of conflicts between hardware/software manufacturers and users, where the question asked every time but still unanswered remains: Who really "owns" the device, the user or the manufacturer?