Well, 10% of the world also uses Macs, but people don't seem to support that as much. Why not? Because it depends on the target demographic. IE6 usage is probably a lot higher in the corporate environments and thus matters to websites targeted at the corporate world, just like how if you're not targeting hip 20somethings, Mac support is unnecessary.
I do consulting and have been in many offices of large corporations in NYC, and I have not seen IE6 on a corporate PC in maybe 5 years. Macs on the other hand are practically neck-and-neck with PCs from what I have seen.
My image of an IE6 user is a 50 year old housewife with a spyware-invested eMachines they bought in 2002. I'm sure in other countries this could be very different but that has been my experience in the US.
