Why don't they let athletes juice? Is it cos it's dangerous or something? Or "unnatural"?
level playing field, in the sense of only what you're born with
which is sticky bc, as noted, genetic differences will always exist, tho sports are obviously more than just a best genes contest (albeit for almost anything at a high level one needs to clear an absurdly high floor of genetic talent to even seriously compete). and there's no single line where a particular supplement or whatever is going to be performance-enhancing in an "unfair" or "unnatural" way.
the "dangerous" argument is usually extended to kids (or minors, anyway) - if kids see role model adults do it they'll do it - because all things being equal, why shouldn't adults who are aware of the risks be able to take whatever substances they want? many sports are inherently dangerous, and so are plenty of activities and substances adults partake of legally.
personally I would never take anything stupid - just whey protein + creatine (incidentally, the only useful supplements - the (legal) supplement industy is a vast ocean of snake oil) because the risk or cost isn't remotely worth the reward, but if it was potentially the difference between a gold metal and 6th place, or landing/not landing millions in endorsements, or whatever, who knows? thankfully I don't have to make that kind of choice