"Roll dice and kick ass!"
What I find a bit more strange is that you can also form a firegroup with a mortar as fireleader, and adding the red fp from some infantry units on or adjacent to the mortar. The whole thing gets really deadly, as it is still against flank defense. This is a good possibility to effectively use those otherwise quite useless 50mm mortars.
I guess the overriding reason for this is simplicity: once you start adding case exceptions of the sort you'd need in this situation, then you've opened the door to who knows how many more of the same. As for rationalising this? Well, the combination of plunging mortar fire coordinated with incoming small arms fire really should be dangerous, don't you think?