I'm not sure it's racism in all cases. Some comments here are of course bigoted, but that's to be expected with the anonymity of the message boards and people's pent up anger. But in many cases, I think, it is related to the fact that when sports are very predictable (one athlete is clearly superior than the others - i.e. Lagat vs. Rupp or Lawi vs. other collegians), that the sport becomes boring. We all (most of us) root for the underdog, regardless of race, because that is part of our culture. And while you have to respect the toughness of Kenyan distance runners and what they've had to overcome in their lives (compared to most Americans), when one runner is so much more talented and wins all the time, it becomes boring. That might be why Lawi does not get as much praise, even though maybe he should.
And in other sports, like for instance golf (if you can call it a sport, which I think you can), even when Tiger was so dominant, it was still very entertaining because he would do amazing things, make shots people had never seen before, shoot phenomenally low scores, but in running, it's just a guy running faster than the other guy to the finish line. So if the same guys win over and over, it's boring, and an upset is far more interesting. It's why people got bored when the Yankees kept winning, for example.