1) There was no testing in baseball for steroids from 1994-2003, to say that he should have known better and been "above suspicion" is wishful thinking. Blame MLB and the MLBPA led by Fehr and Orza.
2) Steroid testing can lead to people getting unfairly punished. Some stuff that is illegal would show up in supposedly harmless substances. Take standard job drug testing. They tell you to avoid poppy seed bagels for a day before you take a drug test for a job, because poppy seeds would show up as Heroin. I am sure that some supplements from GNC would show up as a steroid.
3) MLB wanted the explosion of home runs after the 1994 strike in order to lure fans back into the game. Now the game is tainted and people want "purity" and the pitchers duels and the speedy guys stealing bases. How many people want to watch that kind of baseball? Only NL diehards and grumpy old-timers like Joe Morgan and Phil Mushnick.