Is it fair that Saudi Arabia gets off not helping pay for the damage their citizens caused for 9/11?

yes. It is not Saudi Arabia as a whole that caused 9/11, just a number of individuals in a cruel club. Say the KKK went to somewhere and bombed a school of black children. You wouldn't want everyone to be like "oh you're american. you . you killed a whole school. you're so evil". Because we are individuals it is unfair to group one country in to a whole in this situation.