Microsoft let anyone run software on its OS but was alleged to have deliberately hobbled 3rd party code only if it competed with Internet Explorer. You can't freely run 3rd party code within the OS of a Garmin GPS, either, but you'd laugh if I called Garmin an illegal monopoly on routing algorithms for Garmin GPSes.
My point is, every major design decision at Microsoft now has to go through their legal department to ensure it won't run afoul of any consent decrees. Over the past few years, much of the design process at Microsoft has taken place in a courtroom.
I believe they've only recently been released from the original Netscape antitrust sanctions.
Ask Microsoft how that reasoning worked out for them.