Most likely, GT (or a powerful donor) pulled some strings. Probably, someone associated with GT put in a call to the local DA, who is a friend, to get charges reduced. Petty probably has a clean criminal and driving record, and he didn't crash or kill anyone so the local DA was willing to reduce charges if Petty pleaded guilty. He's probably going to do a lot of community work, and he will probably get the book thrown at him if he F's up again.
For every arrest you read in the papers, there's probably like 10 other instances that never make headlines. I think Key is pretty good at keeping his players accountable. I'm sure Petty's discipline will