They are the only ones who know the truth, but survival is their truth. Bobby does not want the rape discovered and he is certainly not going to mention the murders. His emotional survival depends on the lie. Lewis' and Ed's freedom depend on the lie. They murdered someone, so they will not say anything.
Survival is a theme voraciously examined by Dickey.
It is evident in the works presented here that the methods are not always positive, but when it comes to survival, no matter what kind, instinct is what is in control. Here is a place where the rules must be made up as you go. Survival is something that is achieved by creating the circumstances that are needed. For example, Bobby's instinct tells him to follow the others and then he can forget what happened. If others do not know the events then maybe it did not happen.