In the document, Microsoft says:
We may change or discontinue certain apps or content offered in the Windows Store at any time, for any reason. Sometimes, we do so to respond to legal or contractual requirements. In cases where your security is at risk, or where we’re required to do so for legal reasons, you may not be able to run apps or access content that you previously acquired or purchased a license for.
Computerworld.com points out that this kind of practice is not unusual. Both Apple and Google have similar "kill switches" in place to disable or delete apps from the Apple App Store and the Android Marketplace, respectively. It also points out that Microsoft already has a policy in place for Windows Store app developers that prevents them from using the push notification service to "provide an entry point for viruses, malware, or any other malicious software."