It is shocking because they did it after the investigation had started, which is monumentally stupid.
You can destroy any records you want at any time, unless there's an investigation underway or you have good reason to believe one will be starting. At that point, you're destroying evidence.
The crazy part is the implication that the evidence destroyed was probably more damning than having a judge and jury assume anything reasonably suggested to have been implicated by those chats as true.
I wager they're so used to getting away with shit that destroying evidence probably says less about the crimes committed and more about the piss-poor justice system.
*sigh* After reading some of the other comments, I have to agree. I'm not sure whether to be relieved or even more discouraged. It's a dreadfully boring dystopia.