Because some people have this strange idea that if you can disable an NV provided code part in a game for comparison you should. At the same time the same people have no quarrel using heavily AMD optimized renderers in other games (Hitman, AotS, QB, etc.) for the same comparisons. Makes no sense, especially since HBAO+ runs just fine on AMD h/w, but does skew the performance picture somewhat because it's hard to think of any NV user which won't use NV provided graphical enhancements and some of these enhancement actually do run better on GeForces than on Radeons.