Why are so many people outraged about players who used PED's when they were not illegal.?

Everybody is talking about Sammy Sosa, Mark McGwire, and A-Rod and how they are cheaters for using PED's. But the fact is that they used these when it was not illegal so they cannot be labeled as cheaters because they couldn't cheat because they were not breaking rules. If a person does something and then later there is a law pes saying that one thing was illegal, the person that did it would not be charged because it was not a law before.