Slander is a tort. Not a crime. No one can punish you for slander. The government isn't allowed to incarcerate you for slander, to fine you for slander, and once the court case is over, they do not attempt to put any obstacles in front of you to prevent you from slandering in the future.

You merely have to pay any damages you cause someone else. You truly have a child's grasp of these things.

The court case itself isn't a criminal trial. It's dispute resolution. Someone claims that another caused them damages, the court is empowered to provide remedy to those damages if the claims are found to be true. How does anyone make it to adulthood not knowing this?