The no snitching rule applies mostly to someone who is already in prison and consequences depend on the security level. No one’s going to kill a snitch in minimum security, worst case scenario is that they’ll be outcasts and get into a fight occasionally.

The majority of prisoners are in prison because of a plea deal, not a jury verdict, so they often have to snitch on their accomplices as part of the plea deal (with severe consequences for lying and omissions). In practice the traditional prisoner’s dilemma usually plays out with everyone snitching on each other and everyone getting a deal because the prosecutor doesn’t want to waste time and money on a trial.