It's important to note that this is a clear violation of the exempt employee rule.
With some exceptions, the base pay of a salary basis employee may not be reduced based on the "quality or quantity" of work performed (provided that the employee does "some" work in the work period). This usually means that the base pay of a salary basis employee may not be reduced if s/he performs less work than normal, if the reason for that is determined by the employer. For example, a salary basis pay employee's base pay may not be reduced if there is "no work" to be performed (such as for a plant closing or slow period), and a salary basis employee's base pay may not be reduced for partial day absences. However, employers may "dock" the base pay of salary basis employees in full day increments, for disciplinary suspensions, or for personal leave, or for sickness under a bona fide sick leave plan (as for example if the employee has run out of accrued sick leave).
If the company is regularly doing this, employees classified as exempt could turn around and sue for unpaid overtime wages.