To limit the probability that the horizontal load for the design of firewalls will not take into account the maximum expected forces that occur during a fire, which could lead to the structural failure of or damage to the firewalls, which could lead to the spread of the fire, which could lead to harm to persons on the other side of the firewalls.
To direct Code users to other requirements in Section 4.1. specifying lateral loads on walls [e.g. wind pressure].