Employment Law Questions? Ask an Employment Lawyer.
If you are on salary and are salaried exempt, then you are not actually on salary for 40 hours a week. That's not hour salaried exempt works.
If you are salaried exempt, you are paid a flat rate (your salary) not matter how much you work. You aren't legally entitled to additional pay or overtime, because your entire pay rate is covered by that salary.
The only way this would be illegal is if you have a contract specifically stating that you are to get more for working any time above 40 hours or if you are improperly classified as salaried because you aren't a manager, executive or professional of some sort.
Legally, an employer can change a person's rate of pay if there is no contract specifically stating that the terms of employment can not be changed.
So, that doesn't automatically make what is happening illegal. That being said, your employer sounds like they are operating on the edge of the law, if not on the wrong side of it here. Your recourse is to contact the Department of Labor in your state and file a complaint concerning Wage and Hour issues. That is the only recourse you have. You have to have the government agency tasked with investigating these sorts of claim review the facts and determine if you are improperly classified and/or paid. This is a protected form of complaint, meaning that you can not legally be retaliated against by the employer.