You want to find the cheaters and anyone who taints the purity of the game or the race. (The threat) of testing is a scare tactic that might help eliminate it. It could help the next generation of long-distance runners.
Whenever there's money involved, they pick groups of people to test. The stakes are higher.
(Amateur) athletes may not know what they're doing. The cream they're putting on sore muscles could contain steroids and they don't know it.
