That's not true. Iowa Code 721.2 refers to personal use of state property "to the detriment of the state." If the state is reimbursed, there is a real question about whether there is any "detriment." Nor does there appear to be a clear ISU policy prohibiting what he did.
If you want to be critical about the guy's lousy judgment, go right ahead. But if you're not honest about the standards, it's hard for me to say you're any better than he is.
As a former auditor - you could hang an army of beaurucrats with that language depending on how you felt.
Absent a comprehensive set of policies and procedures, inclusion or specific exclusion in employment documents, etc... there is no protection from BOTH outsiders looking to make ethical hay or a power-hungry administrator who can rely on nebulous or missing standards he's required to adhere to.
If he can use the plane as his private plane and tally 80% of the flights as certification, spell it out and move on. If he has latitude to divert the plane to pick up his family on the way to a game, spell it out. It's easy and it protects ALL parties.
Sloppy work by the university. Did Leath commit crimes? Not even close. Did he give the mere appearance of being ethically challenged? Absolutely.
You shouldn't ever have to say that what you did was perfectly fine then pledge to never do it again. Document it to eliminate all doubt.