the shot clock was introduced in basketball because teams were taking advantage of the rules, which did not require them to shoot the ball.
i understand that steroids and baseball are different. if the league bans "performance-enhancing" drugs, then the new designer steroid is also illegal, even though it is not specifically listed as illegal. sesh seems to know the rules from the times, and i don't disagree. advancing technology (and chemistry) can sometimes make it difficult for the rules to keep up.
</p>ETA - i'm not sure how the stop sign reference works. and i wasn't really answering your question of how something can be against the law but within the MLB rules. that, to me, sounds like the situation where slavery was legal in MS (until recently), but it has been a federal crime since the emancipation proclamation. it really didn't matter whether it was legal in MS.