Yes it is legal.  Since there was no union contract that controlled, or presumably any employment contract or specific company policy, your former employer can hire or fire as it sees fit.  This is known as "at will" employment.  Unless you feel that you have been a victim of discrimination in the workplace, you have no legal claim here.Note:  There can be no discriminamtion for reasons due to your race, religion, age, disability, sex, national origin (some states list other categories as well). You can check with your state's Department of Labor.