MLB also operated using a list of banned substances, and several players circumvented that rule by having new substances developed that were not banned. While one can argue it violated the spirit of the law, it did not violate the letter of the law. Bonds is a prime example of this.
It's also a viable argument that it did not even violate the spirit of the law, since baseball's drug policy at the time was geared to removing cocaine from the game, not steroids. This policy, which included illegal steroids and all illegal drugs, was created to clean up the image of a game tainted by the Pittsburgh Drug Trials, by Steve Howe and his seven lifetime suspensions, by Darryl Strawberry and Doc Gooden, and by the four KC Royals who spent an off-season in prison, among other notable instances. Preventing home runs was not the goal...