The information provided here is not a replacement for a doctor. You shouldn't use it for the purpose of self-diagnosing or self-medicating but rather so you can have a more informed discussion with a professional TCM practitioner.
Because it is a herb specifically indicated to treat lice as can be seen on Stemona Root's page.
Stemona Roots is a Warm herb that tastes Bitter, Pungent and Sweet. It targets the Lung.
Its main actions are: Moistens the Lungs and stops cough. Kills parasites externally.