Electricity always looks for the fastest path to Ground. When Aang took the lightning to the back, the electricity would have traveled through his body and down towards his lowest foot as it searched for Ground.
Not only did the animators include this, they remembered for the future (Sozinโs Comet Part I).
Sue everyone. I don't care. Sue the airline, the airport, the agents, cause TSA agents subjecting a severely disabled person to this is physical torture, and now his condition has worsened because of their lack of compassion and commonsense.
This was completely unnecessary.