Ban Drugs in Sports?

For those interested, I pose this as a question for discussion. Is it appropriate for sports leagues to ban the use of particular types of supplements or chemicals?

As a libertarian, I think a private league can set such a policy with legal and political impunity. The question I am wondering about is if there are good (and enough) reasons for a league to set such a policy. (leaving aside public relations reasons or do-it-yourself-before-government-steps-in reasons)

My skepticism towards such policies arises namely out of its apparent arbitrariness. Why are some substances banned, but not others? Why steroids and HGH, but not protein shakes and other diets/supplements that allow for great muscle growth and recover time? Most would, I think, argue that the former are dangerous and set bad examples for youth. Yet, the former under physician care can be used safely, no doubt with risk. The 'children' argument has some weight, but again, targeting these substances and not others seems arbitrary. It can be unhealthy for a teenager to live on a diet of protein shakes and bars, but that is not brought up as a justification to ban these.

Certainly there is a continuum here from highly risky to less risky, from highly advantageous to only moderately so. The question seems to be at what point one thinks the risk is too great or the advantage too great (and thus unfair).

Another thought: is there a difference between using said substances to maintain condition and recover from injury versus using them a performance enhancement?

And why the focus on substances and not other performance enhancing activities? Excessive training can be unhealthy (and thus fall into the category of bad example) and give one a performance advantage. Some trainers and conditioning approaches are far superior to others and so provide for performance advantages. Why not also ban chiropractors and physical therapists from aiding in injury recovery and prevention? Why does MLB allow for voluntary Tommy-John surgery but not taking a pill that might accomplish similar results?

Thoughts?