Telling people how they need to behave doesn't improve behavior. It's not like telling them what they did wrong will make them have an epiphany and understand the sort of incomprehensibly bad posting they're doing. You have to be pretty consistently bad to get irrevocably hellbanned, and telling people what they did wrong instead of banning them, if it has an impact on their behavior, would tend to make them skate along at the boundary and lower the quality of the forum anyway. If those people had good comments to make, they'd have already been making them.