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It makes some degree of sense to use leaded gasoline in small engine aircraft - if something goes wrong with the engine (leaded gasoline reduces knocking) it's a whole lot worse than in a car.
Jets and large planes use jet fuel, not leaded gas. The twin engine Cessnas aren't really contributing much to the issue.
For starters it's not exactly a new leak, and it's not the first time something like this has happened either. Leaded gasoline was known to be dangerous before it was even released and they pushed for it anyways. We still use it in aviation to this day as well.  
If we don't even prosecute people in charge for things like the GM ignition switch deaths, there's little hope we'd prosecute ExxonMobil for hiding climate change studies. We need greater penalties for corporations who harm society like this but that will never happen with the modern Republican party unless Democrats gain a supermajority.