Barozi on 04 July 2016
Because they assume they will get more money from people buying PC games that aren't willing to pay for online than from people buying PC games and willing to pay for online.
Your mistake is that you think that all the money from XBL Gold is going straight into the servers. Completely ignoring the fact that Microsoft will get an additional source of revenue from selling PC games, which can then be put right back into improving the online service on both PC and Xbox.
So yes, even if PC only gamers don't pay for Gold, they are still contributing to improve the Xbox Live service.
