It's because there's money to be made there. Valve disclosed today that in the week since launching its Steam service on Mac computers, 11% of all purchases on the digital shopfront have been made for Apple's platform.
That's a massive boost to the company's bottom line, and also helps lend the Mac some credibility in the eyes of those who believed its install base was too small for Valve to bother releasing games for.
Sure, you could argue that it was a first-week rush, frustrated mac gamers gorging themselves on quality titles, but this is Steam we're talking about. New games will come. Old games will get bundled, and go on sale, sometimes for ridiculous prices. You'd think most people signing up, downloading and buying something will be in it for the long haul, just like most PC gamers seem to be these days.
While we're on the subject, Valve also revealed that, while Portal is running a little slower on the Mac, the Apple version is also five times less likely to crash than its PC counterpart.