It's acceptable because a tip is a bonus. She isn't entitled to it and she shouldn't be expecting a tip. She's doing her job and earning her wage.
No, she is not entitled to it. You're right. But in this country tips are expected. Server wage structures are designed so that servers make money from tips, not wages. Yes, they will make min. wage regardless of what tips they pull in, but they still have to report a set percentage of their tips to the government for wage withholdings.
