As I understand it, Paterno is not being charged because the DA did not find he did anything against the law. As is PSU code, when he was informed of the accusations, he went to his superior and told him. It's clear that the AD broke the law and is going to be put on trial.
We don't know what Paterno did, if anything at all, after that in relation to the accusations.
I assume he brushed it off after telling the AD and believed the school would investigate or take it further and (correctly) notify the police. That's where my issue comes from. You're Joe Paterno, how do you just (apparently) wash your hands of the situation and move on like nothing happened, friend or not. At best, it's a false accusation. At worst, it corrupts the program and stains JoePa's reputation.