You are correct, and while possibly implausible, certainly not to be ruled out. In fact, there are some ways to goad your opponent into making the enabling attack. More importantly, the same restriction on regions, if you will, applies to all teams. Look for ways to exploit that, for example, if you have one area that is essentially off limits to opponents in order to prevent your team from taking a region, then that effectively gives you a map area you no longer need to defend. Furthermore, the opponents will not know if THEY need to defend against possible attacks from that area.