The monetary reward that the WL's get from offering a pussy slide service and the customer retention is worth the risks maybe? I don't know the exact reasons because I've never asked a WL before.
Customer retention for sure. It's part of their repertoire.
RE the risk of catching something, the girls that do it but not BBFS are more worried about getting pregnant than STIs. That's prolly the main reason they don't do BBFS.