You must be a tip eligible employee. That class of employee is paid a base wage as low as $3.25 and hour, and if the tips you receive do not add to that wage an amount equal to the minimum wage in your state, the employer is required to make up the difference. Stupid law, requiring customers to make up the pay of the underpaid server employees.
We spend a good bit of time in France, and there, tipping is a thank you for excellent service. It is not customary or expected. We usually leave a couple of Euro as a gratuity, to say thank you to the server, but it is not expected.