Why did the painter paint the wall for free? Because it was on the house.
I assumed it so, but since the morale check for assaulting troops is optional, I wasn´t sure...
I think it demonstrates that they had the optional rules in mind when writing the terrain rules. Under the basic game rules, units attacking in close combat never test morale, so there would be no reason to specify that the bonuses or penalties for morale based on terrain only apply to the defender, unless you take the optional rules into consideration.