Yep, agreed. It'd be interesting to test the legal waters for recovery of lost revenue through consumption of perfectly legal (and more to the point, acceptable in aviation) medication or substances.
And think of the tarnished company name when words gets around that "those two charters didn't go ahead yesterday cause 3 out of the 4 crew tested positive for banned substances". Just like a rape charge - the allegation is often as bad as the conviction. 3 out of 4 is an extreme example - but quite possible.