Because human eyes are able to see that 20FPS difference... RIIIIIIIIIIIIIIIIIIIGHT....
Actually they can.
Also that's not really the point. I also don't think they want to bash anyone, and if I had to find a target for some perceived bashing, it would be Valve, for not being capable of a perfect port.
Still, it seems like the performance differences are not as big as some other ports, so all in all, I'd still be pretty happy to be a Mac owner who plays with Steam (since I doubt that someone would buy a Mac for games, so the rapidly extending library is more like icing on the cake).
Movies are presented in cinemas about 25FPS (cant remeber the exact number right now) And humans do not notice any hickups... So 60 or 90 FPS is for humans the same as 25 FPS... the only difference in games, in this FPS difference is, that the mouse is better responsive and has better performance with higher FPS...