> Some of that is almost certainly confounded by the fact that many asthma medications - most notably salbutamol - are performance-enhancing drugs; an asthma diagnosis allows an athlete to take drugs under a therapeutic use exemption that would otherwise result in a ban.

I think all of it might be explained by this. At least to me it was always "common knowledge" that many athletes find a doctor willing to diagnose them with asthma so that they can get the medication.