Menzies Aviation was fined by the California Division of Occupational Safety and Health (Cal/OSHA) following the death of Cesar Valenzuela, 51. He was killed after being thrown from a baggage cart at Los Angeles International Airport (LAX) that did not have a functioning seat belt.
The Service Employees International Union expressed concern because Mr Valenzuela was the fourth Menzies worker to die following workplace accidents at California airports in eight years. There had been three deaths at LAX and one at San Francisco International Airport.
Cal/OSHA fined Menzies nearly £60,000 ($95,000) in 2013 for using unsafe practices, including the type of safety violations that can cause serious harm or death. One of the 23 citations was for failing to comply with state seatbelt rules.
More than 100 Menzies workers went on strike in May 2012 to protest at working conditions at LAX, citing concerns about the serious health and safety risks they claimed they faced on the job.
Investigators determined Menzies' safety policy did not require seat belts and discouraged use of safety belts in parts of the airport.
Christine Baker, director of the California Department of Industrial Relations, which includes the Division of Occupational Safety and Health, said of Mr Valenzuela's death: "This fatality could have been prevented with a well thought out and implemented safety plan, as is required for all worksites in California."
Menzies and other aviation service companies have contracts with airlines to provide cabin cleaners, security personnel, wheelchair assistants and baggage handlers.
The case brought against Menzies prompted union officials and service company employees to renew calls for improvements in working conditions at LAX, the USA's third-busiest airport.
Menzies officials said the firm would appeal against the decision, adding that they disagreed with the findings and were disappointed that investigators did not consider the company's 147-page response.
They said: "Menzies is confident that, through the appellate process, our company's ongoing commitment to workplace safety will be made evident and the citations will be overturned or revised."