Using steroids to enhance performance has been illegal in the United States for a very long time. This is like arguing there was no rule against knee-capping your opponent in skating so Tanya Harding boyfriend's knee-capping of Nancy Kerrigan was within the rules.
Myth 2) Steroids don't really help performance, at least not in baseball
Of course no one makes this argument any more. But it was a favorite for a while. Occasionally it gets resurrected in arguments about admitting Mark McGwire to the HOF. Of course steroids by themselves don't have any effect, but used in the context of an exercise program they can and do have a dramatic effect.
Myth 3) Steroid use started in the late 90's
While we have no idea who first used steroids in baseball, their use in sports started at least in the 1970's. Given there are hundreds of players who see their dream of being a major league player disappearing and the difference in salary between being a AAAA player and major league regular, you have to assume they started getting used in baseball pretty early. In the 1980's guys had started to show up in major league camps with 30 pounds of additional muscle from "hitting the weights" in the off-season. We started hearing about the benefits of "modern weight training". Its not clear that any of those terms were more than euphemisms for body-building techniques using drugs to enhance performance.
Myth 4) Steroid users are obvious, because they will immediately pop up to look like Arnold Swarznegger
While Swarznegger and other body builders made steroids popular, the effect they have depends on what exercises go along with them. Roger Clemens never really had that "steroid build", but his workouts were focused primarily on building leg strength. The fact that a guy doesn't look like Hulk Hogan does not mean he isn't using steroids. On the other hand, if he does look like Hulk Hogan its not unlikely he is.
Myth 5) Jose Canseco's accounts couldn't be trusted
This is arguable I suppose. Maybe there are other reasons to mistrust Canseco. But all sorts of people who attacked Canseco have turned out to be liars themselves. While not every one of Canseco's accounts have been confirmed, there aren't any that have been credibly disproved either.
Myth 6) The ball was juiced
This was a claim made repeatedly during the steroid era to explain the sudden power surge. We now know there was another explanation. Its time to put the juiced ball myth to bed.
Myth 7) Steroids only help hitters
Roger Clemens is the poster boy for why this isn't true. But it is doubtful that he was the only pitcher who used. There are people who argue steroids have an even bigger impact on pitchers than hitters. Although the physical signs may not be there. All those aging pitchers who "kept themselves in shape" may have been doing more than eating right and getting proper exercise.
Myth 8) Sudden jumps in production are common in players over 30
Most players have had their most productive offensive year before age 27-28. If you look at the average numbers there is a sharp improvement in performance up until that point, a leveling off and then a decline after age 30-32. Of course averages don't apply here. Some players age more gracefully than others and outstanding players have maintained high levels of production into their late 30's. But the jumps we saw in guys like McGwire and Bonds are extraordinary, not only in the size of the increase but the time of their career when it happened.
Myth 9) Lots of players develop from singles hitters to home run hitters as they mature and put on muscle. So big jumps in home runs are to be expected.
There is no doubt that home run power develops later as players put on their "man muscles", as Torii Hunter refers to them. We saw that again with Joe Mauer. But that power potential is usually obvious. The players have the frame and gap power that projects to home run power. When a player hits 25-30 more home runs in one year than they ever have in the past, that is not a normal development pattern. It may have happened without the help of drugs, but not very often.
Myth 10) Baseball has cleaned up its act
While there is now steroid testing, it is not 100% effective at catching people who use steroids. In addition, there is no test for HGH (which, like steroids, some people now claim doesn't actually help performance).
The larger problem is that baseball has not really rejected steroids the way they did gambling after the Black Sox episode. The players who were associated with that scandal were barred from baseball for life - in fact even after they died. Players who used steroids have been allowed to continue to play as if nothing happened. Admitted steroid abusers like Mark McGwire are hired to coach young players and held up as role models for the next generation. While apologizing for using steroids, McGwire talks about how they helped extend his career. Tony LaRussa and Mark McGwire keep blaming everybody but Mark McGwire for his problems.The most recent person to "victimize" McGwire is his brother. McGwire claims his brother is lying too.
For baseball, steroids is a PR problem. The clear message to players - don't get caught and if you do, apologize and it won't damage your career. If you expose your teammate's use, you will be ostracized and publicly defamed as a liar like Jose Canseco. The goal is to sweep the problem under the rug and hope it will disappear from public view. But that is dangerous both for the sport, and for young players faced with their career ending who hear the siren song of Mark McGwire, "at least it extended my career".