After a celebrity scandal like Armstrong's, sponsors like Nike weigh their options

At least five other companies have dropped Armstrong like he was a hot potato after Nike did.  But why is this time so much different from any of the other scandals that Nike has gone through with their athletes?  For one thing, those other cases- Tiger Woods cheating on his wife, Michael Vick with his illegal dog fighting- these scandals didn't affect the athletes proformances.  Armstrong cheated, though.  Nike wants to be associated with great athletes, not preformence enhancing drug takers that only win because they are doing things that other teams wont do because (1)its unethical and (2)its not their own personal strength thats taking them through the race.