Why not just create a "system On/Off" scenario? No matter what the customer chooses to watch the Cable box turns ON, and when he turns the system Off the cable box turns OFF?

With Crestron you can track the state of the system via logic (buffers, interlocks, etc.) or device feedback (from TV, Receiver, DM Route) and ensure that you are only issuing the Power command when the room needs to be ON/OFF and not during source switching.

If you are getting feedback from a device this would be easy within programming
TV Power feedback = ON - One Shot cable box power
TV Power feedback = OFF - One Shot cable box power

Logic wise, there are many ways to achieve this. I would probably use an analog initialize and analog equate with a one-shot, but you could do this with interlock and buffers too, and I am sure there are other ways to do this that may be better than the above two options. Depending on the Crestron Program if you are using crosspoints this would track source selection for the room and you could base the cable box power on the source selected.

All this is assuming that the cable box is specific to the room and audio output is not being used for multi-room music or shared with multiple TVs.

No this doesn't solve the problem if someone were to physically press the power button on the cable box, but it sounds like this isn't happening now so probably won't be happening in the future either.