I'm not an expect, but I imagine no IS is used because panning and IS don't mix. There is a newer form of IS (mode 2) that allows panning along a single place (horizontal or vertical). But my experience with shooting a single air show is that much of my camera panning was at an angle as the planes went overhead. I don't think mode 2 would be able to compensate for that and you would end up with the majority of your photos blurred.