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Unfortunately yes, it is legal. There is nothing illegal about an employer acting in punishment unless they are retaliating against someone for making a complaint that is protected (like a complaint of discrimination based race, religion, gender, age, disability or FMLA use...or if the employee is a whistleblower under OSHA, the Department of Labor, Sarbanes-Oxley or some other specific statute).
As a salaried employee, it also isn't illegal to make hours mandatory....even that number of hours.
Not under federal law, as it pertains to salaried employees.
Your state does have its own law that is called the One Day in Seven Rest law, but it specifically exempts salaried employees under the FLSA too.