Let's remember that a panicked model is not out of the game. It just can not aim, wait, MP, channel, advance closer to the unit that panicked it, and is -3 on its CC, RC, & LD. I could see having to spend the action to get the unit on their feet if they were previously in MP but, they would only need to retreat if they are broken.
Let's also consider this in game terms. I just did well enough to cause your unit to panic. So, I should be rewarded not you. Why should I have to wait for you to activate that unit (which under most circumstances will be your last activation) before you loose your MP status. If you loose your MP status immediately then my other units can now have an easier time shooting at you.