The general feeling in the rest of the thread is that this was to obfuscate these hosts and prevent them from being blocked by malware. However, there are no non-MicroSoft hosts listed, giving a competitive advantage for MicroSoft's anti-malware tools over other brands.
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Dave Korn announced on the Full Disclosure and Bugtraq security lists that Microsoft is bypassing local lookups for some hosts, meaning that you can't locally block some sites through your HOSTS file. All of these sites are MicroSoft controlled sites.