I am assuming that the reason that you do not want hourly workers putting in time on company-observed holidays when exempt employees are not at work is because the hourly workers would not be properly supervised and/or as effective in their roles with key personnel not at work. This is a legitimate business reason for what you are doing.

Management has an absolute right to schedule work days/hours and no obligation to ensure that hourly employees get their 40 hours in during weeks when holidays occur. So, you are doing nothing illegal in simply closing your operations on company-observed holidays and/or only having workers report for work if instructed due to you having a legitimate need for their services on those days.

From the way your question is stated, I assume that you do not have contracts/agreements/written policies or standard practices to the contrary of what you stated you are doing. Contradictory contracts and written agreements would need to be renegotiated. However, in the case of a policy or practice, you have the right to make a change and promulgate it whenever you wish to do so.