From wikipedia:
"The United States of America federal government requires a wage of at least $2.13 per hour be paid to employees that receive at least $30 per month in tips.[4] If wages and tips do not equal the federal minimum wage of $7.25 per hour during any week, the employer is required to increase cash wages to compensate.[5]"
It's phrased to sound like a top-up but it's theft in my opinion.