They might have diverse sources of income, but the majority comes from Windows and Office. For at least the last decade I was watched as many new companies have been run with mac/linux. This doesn't mean much when they are little five person startups, but as the years go by those startups grew and now we have some very large companies that are not based upon Microsoft and plenty of medium size ones. This creates a feedback look of employees that don't train people how to write microsoft a check each quarter, but how to use linux servers etc. It is hard to ignore this as a long term trend. In fact it is so weird/odd/rare that stories get posted here on HN about the reasons why startup X choose to be a MS shop. Saying I am going to use github, amazon and google apps wouldn't get any interest, it is simply what people do these days.