Well most players don't headbutt each other. Duncan did, met prison, iirc McStay wasnt injured as such.
The charge against Ferguson was an absolutely ridiculous one. It's pretty much the only example of a footballer being jailed for anything done on the pitch, ever.
However, even under these extraordinary and frankly, laughable circumstances he was only given a prison sentence because of his previous convictions and the fact that he was on probation.
So stop being a fucking pussy and save your indignation for the truly horrendous crimes, like Suarez against Mirallas, like Roy Keane on Haaland, Nolan on Anichebe and so on.