I stated unequivocally the PC would die - yes, I was aware of the rule - yes, I guess this is a ruling outside the rules, and therefore, a house rule that was unannounced. However, I countered, the player was exploiting player knowledge of the rules to benefit his PC.
This
is a case of blatant meta-gaming... on the part of the DM. You made your ruling (that this fall is fatal) based entirely on factors external to the game world. There's absolutely no reason why the ground would take the character's
intent into consideration, when determining how hard they hit. The only things that could possibly affect such a thing are those factors which are actually internal to the game world - the composition of the falling body, what it lands on, and possibly the technique with which they fall.
I have nothing against a blanket house rule regarding the lethality of falling, but it does raise inconsistencies with other parts of the system. I mean, falling from twelve stories up is pretty lethal, but it isn't significantly
more lethal than getting shot with ten arrows. In my experience, this line of thinking is a short step from claiming that the rules don't actually model the things that they're clearly attempting to model, even though they spend quite a bit of effort in quantifying the various relevant factors; which raises the further question of why anyone would use
this ruleset in the first place, if it doesn't even tell us what's actually going on in the world.
To answer your question, though, the purpose of the limit on falling damage is to reflect that mundane physical hazards really
aren't a threat to creatures of a certain power level. It would be inconsistent for any being to die from a simple fall, if they could otherwise withstand taking a meteor to the face. And, whatever else you may try to do with them, Hit Points are still
the metric which is used to govern tolerance against injury.