There is a long history of ethical behavior being a contingent requirement of the professions. So, as our economy involved more and more specialization we require our doctors not to be butchers, our dentists not to sexually exploit their patients. And for our athletes not to be beholden to the gambling and mafia elements. Yep, its a minimal requirement of civilization that we hold people to certain moral and ethical standards.
Or, you could have sports be pro wrestling.