It is completely legal. An employer is free to set dress codes, including that an employee show up and depart work in uniform. The employer does not have to pay the employees any additional compensation for showing up and departing in uniform. This is legally no different than a bank or law firm requiring that employees show up in suits and ties, rather than coming in causual clothing and changing at work; in both that case of the bank or law firm and the case of your employer, the employer can determine what clothing the employee must wear to/from work.