IP is not a hard evidence to frame someone.
You know, PCs all over the world change IP dynamically and too often, tracking someone isn't impossible, but you have to target him and watch his moves, closely.
Due to the architecture of internet, when someone connects to a server, this request hoping around nodes all over the world to reach its destination.
SO the KEY
Due to signal hoping and proxy connections (faking client's ip with someone's in the middle), you can refuse to admit that you did anything,
you can just pretend that you are innocent, you know nothing and give the impression that YOU are the first who has been hacked(hackers used your PC as a proxy).
Terry Colby had a significant position at a corporation that knows how to use internet technologies, so Colby couldn't use excuses like that.