So, yes, if performance enhancing drugs aren't against baseball rules, it wouldn't be cheating. Since they are against baseball rules, though, it's cheating.
There's plenty of other examples of this in other sports. Basketball has a lot of them. They created the 3 second violation because Wilt Chamberlain would just stand in the paint and make it impossible for anyone to get inside. Was he cheating when the rule wasn't there before? No. The difference is that EVERY player could use a corked bat when it wasn't illegal. If only one guy thought of it, that makes him smart, not a cheater.
Performance drugs aren't against baseball rules. Steriods and other such drugs are illegal, but there are still MANY drugs that are legal that players can use that do the SAME effect as steriods.
Basketball is a little different b/c everyone gets the same advantage. Everyone could of corked their bat, but I'm sure everyone didn't know they could. BTW, Bill Russell was also a big reason for the 3-second violation. Just had to give my Boston boy some credit.
I do see your point that it wasn't against the rules, so it wasn't cheating, but it just makes me see his 600+ hr's as not as good as others. This also all depends on if he did cork his bat.