If you look at wikimedia's stats, the amount of linux and mac users seems to be relatively on par. The "1%" commonly quoted is from a paid stat counting company having customers such as microsoft.com.
In any case, my point is that quite a couple of the steam games, e.g. world of goo already have linux ports. If they add a linux client, those that ported their games to linux (which especially amongst indie developers is getting more and more common) will already be available for their steambox.
Yeah, mac doesn't have a ton of games, but it has quite a couple. It just seems like a good idea to have ported and ready games available in addition to the ones they will inevitably get if they do make a console.