Drugs are less banned, than certain drugs are allowed. The point is that, even if a drug doesn't have any ability to change the result of the race, they don't want it found in the horses system at detectable levels post-race.

For example, virtually all the clenbuterol positives trainers get are technical positives, but the drug is present at such low, non-efficacious micro-doses that it couldn't possibly affect the race outcome. Impossible.