So in real life if someone attacks your country, do you use "free choice" and fight for the enemy?
Masters are supposed to be the representatives of their chosen elements. Switching sides at a time of war is wrong on many different levels. Keep in mind that how a Master did his/her job as a Master will have an effect when other players try to take his/her title away, so helping the enemy might not be the smartest thing to do.
Besides, this is not a 100% mono-deck event so players have more options in deck building.
What if you live in one country but consider yourself from another? What if you always use one element and feel like trying another?
In real life, if someone attacks my country, I figure out why before I do anything.