Doping hasn't just been a problem among Kenyan marathon runners. Russia's Liliya Shobukhova received a two-year doping ban last April. She won Chicago three straight years from 2009-11.
Just the tip of the iceberg. Eight or nine "B" level athletes from Kenya tested postive recently. They were the sacrificial lambs, not unlike some of the Jamaican sprinters last year. With only a 2 year ban and practically no way to get the prize/appearance money back the incentive to cheat is pretty high.