There are lawsuits flying every which way over absurd things the patent office has granted patents for - so if you hold the patent for something ridiculous and widespread, you not only prevent someone else from getting it and suing you, but it's a bargaining chip when someone comes at you with another patent violation. They say "we'll sue you because we hold the patent on 'an electronic way to display text'" and you say "if you do that, we'll sue you because we hold a patent on 'a tactile alphanumeric input device.'" And so instead of a costly legal battle, you've won yourself a stalemate, with only the price of a few patent applications.
Not to say that this is an appropriate way for the system to function, and not that big companies shouldn't be lobbying to fix the patent system (I believe some of them are), but I feel like M$ is just playing the hardball they need to play to protect themselves.