If players are able to perform better in the 30s and 40s perhaps the ‘side effects’ are over-rated.
In any event - why is it legal for a woman to mutilate her body with silicone but illegal for a man to want to enhance natural muscularity for performance and or vanity?
It’s not illegal (generally) to ‘enhance’ muscularity, providing controlled-substance laws are obeyed.
The question concerns professional sports and whether they can have enforceable rules to protect themselves from lawsuits if for no other reason. They can.
Not many familiar with side-effects would call them ‘over-rated’.
http://www.thestlcardinals.com/SteroidsclaimthelifeofKenCaminiti-Wasitworthit.html
http://www.sptimes.com/2005/02/21/Sports/_Roid_rage.shtml