Because he knows something and chooses to keep his mouth shut, period. Which is actually the smartest thing any of them have done.
But what I dont get is, there are only two possibilities, imo, regarding Misty and the truth. She knows what happened, and is very culpable in what happened (and it would keep her in prison or worse for a lot longer than the drug charges) or she really has no idea what happened. There is no way I believe she would sit in prison for this long for someone else. So its either her being directly involved in some major way, or she really has NO idea what happened. Think about it. I dont think any of these players would sit in prison for another. So, if they arent directly involved then they dont know what happened. I dont believe there is a single player in prison who wasnt involved, but KNOWS who was...sorry, it just doesnt make sense.
That being said....if Timmy knows what happened, he doesnt talk because, why should he?? Hes not in prison...so he actually has the choice of not being a rat, which he may be exercising. If he is involved, and anyone in prison knows he was involved, why hasnt the finger been pointed at Tim more than it was?? Why all the fingers on Joe? It just so weird. So, my conclusion from all this may be that Tim (who took off so quickly) was involved directly with the players in prison who were involved, or he knows, when others may not know. I think Tim could hold a bigger key to this than Misty does, perhaps. I dont believe Misty was home that night with the kids and someone knew it.