In lots of places, they aren't liable, but many people aren't aware of this, and/or aren't in a position to fight it. It's one of those things that vary by state, but in NY, for example, it's definitely illegal.

Employers are only allowed to deduct certain items from an employee's wages, such as taxes, insurance premiums, union dues, etc. They are not permitted to charge employees for breakages, cash shortages, fines or any other losses to the business.

Restaurants tend to employ younger workers, and often hire undocumented immigrants, so it's easier for them to pull one over on their staff who may not know their rights, or be scared of retaliation if they do try to insist on their rights being respected.