The vast majority of those people had a facebook account anyway. Using Skype to push Microsoft accounts gets people to create MS accounts that they didn't have. I imagine there's a selected-by-default checkbox in that process along the lines of "Send me exciting news an announcements about Microsoft products!"
Soon to be followed with "HEY BUDDY, DID YOU KNOW OFFICE 365 WORKS ON ALL YOUR DEVICES? AND IT'S 10% OFF!"
It seems obvious to us, but we are an exception. Most of the people just don't care.