It's not targeted at real people who have to make a choice about which to download. It's been pretty well established that the only people still on IE6 are stuck behind IT policies that don't allow for upgrades. It's just a political statement, then, an attempt to drive home the point that IE6 is an ancient and decrepit bane on society and technology. The more sheer embarrassment that you can pile on top of these IT departments, the more frustration you ladle into the end users that they can direct into getting their company policies changed (rather than, hopefully, directed at you), the better. I think showing a wider variety of actually viable browsers makes that point better (hell, throw Opera in), because it makes it obvious just how far behind the rest of the world has left IE6.
- IE8 is there because it is a clear upgrade path
- FireFox is there because it has well known brand recognition
- Chrome is there because they want to push their own tech
What would Safari there gain? It would simply be one more choice. I think the three are already too many.