I totally see that IE6 & 7 are not worth the dev effort, IE8 is probably border-line, but IE9 is a good browser and people seem to be vastly exagerating how much work goes into getting IE9 to look and work like other browsers (i.e. not very much).
So not supporting IE9 seems a bit lazy to me, and I have to say, a little "religous" (they seemed to have ruled out IE10 already without doing any research into why). Also find it strange that they actively block it rather than just warn people about it being unsupported.
However, I will cut them slack because the target market are creatives, and it's an admin site rather than a consumer level site, so probably few IE users here. But others here seem to be extending this idea to the public web, which seems like a mistake...