My understanding of copyright law or trademarks is that they must be enforced to be valid. If a company is granted a trademark and then does not enforce it (by requiring others to not use it) then they can loose the trademark -- it becomes public domain.

I would expect that Microsoft is trying to protect this from happening (the lapse of their trademark from becoming public domain). Some might argue that this has already happened (from the many people that are actively using the logo for many years without notice) -- but that is an argue for lawyers and courts and people that have nothing better to do with their money than give it to the lawyers!!