Oh it's definitely a slippery slope, and I don't disagree, but, he's a footy coach, not a biochemist. Plus, Dank potentially could have been giving them substances of self serving means. If players perform better fitness wise it probably means more money for him either in bonuses and/or future contracts. At the end of the day, Hird would be pushing the proverbial up a hill trying to fight EFC in court. But it is possible (not likely though) that he thought everything was legit and above board.
He wanted everything to be above board, he believes everything was above board.
Essendon cannot logically at this point sack him on the basis of anything other than him appealing when the club isn't.