I can only speak for the state I live in, Massachusetts. Waitstaff don't make minimum wage here so they rely on trying to provide good service and receiving a tip for it. 20% seems to be the norm unless the service is poor. I never really thought about it, but maybe it is an American thing. We tip waiters, bartenders, barbers, the newspaper guy gets a monetary gift at Christmas, the postman gets one too, guy carries your bags into a hotel gets slipped some money. Maybe it would be better if the gratuities were factored into the overall price.