The air is being blown onto the heat sink and going off to the side, why do you think heatsinks are designed like that? Wouldnt you think that if the right way to do it was pulling air AWAY from the heatsink, they would have made it do that?
Try for yourself. Take somethingk that is hot, very hot, constantly getting hotter if not actively cooled. Now take a BIG fan and put it next to it, blowing at it, and see how cool it gets. Now turn the fan around, sucking from it, how cool does it get?
If you put a fan in the window of your bedroom to cool it off, which way does it work best? Blowing air into the room, or taking it out? Same theory, same laws of physics, same physics apply.