Steam announces support for Mac and a few months later Microsoft decide to focus on PC gaming more after totally ignoring the market for years.
The vast majority of Microsoft's profit comes from the Office/Windows lines (I think its the best part of 70% if I remember correctly). Everything else - XBox, Bing, Sharepoint, Exchange, Azure, BPOS etc. etc. etc. are but footnotes in comparison. Of course they'll do everything they can to protect the Windows (and hence Office) monopoly.
As a culture, Microsoft seem genuinely incapable of any innovation. This is just another blatantly transparent reaction to try and claw back a slice of yet another market segment they've been too late to the party on.
slayerN - PC gaming has been very rapidly loosing relevance. It's a lot cheaper to develop for and support consoles and you have less of the problems with game experience control and piracy. The most recent shareholders reports from ATVI and ERTS make it abundantly clear PC's are fast becoming a platform of diminishing returns in the case of many titles. Its simply not worth doing the extra work to release many games on that platform unless there is a very good expectation of high sales figures.
Interestingly - when MS did make games for Windows they did some pretty good ones - Fury3, StarLancer, Halo, FreeLancer, Mech Warrior, Age of Empires. Midtown Madness etc.