We're talking about the top 90% of earners so it makes sense to enlarge the right side of the graph. They're trying to illustrate the huge changes in income for every percentage point. A linear axis would be basically vertical at the right end.
I think a vertical line would show the point more clearly. The idea is that there is a lot more change at the top end than the rest, and a vertical line does that.
Resolution does become an issue (I agree that showing the change from 99.5 to 99.9 is good and a linear scale makes seeing those points harder). A way around that is to have the graph scroll left to right -- which should be easy given that it is an interactive JS/flash graph widget.
