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 from the building to an adjacent building, which could lead to damage to adjacent buildings.
To direct Code users to other requirements in Section 4.1. specifying lateral loads on walls [e.g. wind pressure].