No, it is not. Federal law specifically prohibits this.
QUOTE:
"Retention of Tips: The law forbids any arrangement between the employer and the tipped employee whereby any part of the tip received becomes the property of the employer. A tip is the sole property of the tipped employee. Where an employer does not strictly observe the tip credit provisions of the Act, no tip credit may be claimed and the employees are entitled to receive the full cash minimum wage, in addition to retaining tips they may\should have received.
Service Charges: A compulsory charge for service, for example, 15 percent of the bill, is not a tip. Such charges are part of the employer's gross receipts. Where service charges are imposed and the employee receives no tips, the employer must pay the entire minimum wage and overtime required by the Act.
Tip Pooling: The requirement that an employee must retain all tips does not preclude a valid tip pooling or sharing arrangement among employees who customarily and regularly receive tips, such as waiters, waitresses, bellhops, counter personnel (who serve customers), busboys/girls and service bartenders. Tipped employees may not be required to share their tips with employees who have not customarily and regularly participated in tip pooling arrangements."
ENDQUOTE
An argument could be made that retaining part of every employees tips for a holiday party - for ONLY tipped employees - constituted a "valid tip pool", but it's a stretch. The moment the owner stuck her hand in the jar and took some of the money for her own use, though, the issue became moot. That's your money, and you can demand it back.
Richard