Wife's boss is on a power trip. Is this legal?

My wife works in a restaurant, and the power-tripping manager has instituted a new policy where all shift changes must be approved by management. I think that is reasonable enough, but they're also asking the originally-scheduled employee why they are switching shifts, then approving or denying based on the answer.

For example, her coworker (Tom) wanted Monday afternoon off, and Harry agreed to cover the shift. The manager asked Tom why he wanted Harry to work for him, and Tom said, "I have a softball game." Manager denied the shift change because it was "unnecessary".

Is this legal? I feel like if you're able to find someone to cover your shift, you don't owe management any explanation why you need the time off. How should my wife approach this situation? Colorado, USA BTW.