Are employees covered under the "no purchase necessary" laws, when the employer runs a contest for employees only?

Not-for-profit retail company is running a contest to increase sales. The contest is open to employees only. Any employee purchasing items totaling $10 or more is entered in a drawing to win a $100 gift card. Each $10 purchase gets entered into the drawing. Purchase is necessary. No provisions are made if you do not make a purchase - you just don't get a chance to win. The company has over 250 employees and is located in Central Massachusetts. Is this legal?