I can see how some might think that. I checked into it and it has to do with using a unified login system for all MS services. The number of Live accounts that could possibly be added via a bug system is incredibly small compared to our other services. Also, consider that Google requires a Google account to submit bugs for Chromium. I'm sure it's for the same reasons as the number of Gmail accounts created by devs & bug submitters is incredibly small compared to their overall consumer base.