[Edi]

Be hard to restrict the fire command. You can mix archers and infantry and archers can lose their arms, in which case they lose their bows. If the availability of the fire command was dependent on the presence of missile weapons, it would lead to all kinds of micromanagement problems. It could also have all kinds of other related bugs attached if that were the case. So my vote is that it was a design decision as well and in my opinion a good one.

After all, a unit that has received mistaken fire orders but can't execute them will default to attack anyway, so no harm done on that score. And if the whole strategy depended on the non-missile units being missile units, well, that's the general's fault, isn't it?