This is a distinction I just don't get. Internal setting consistency would tell me it matters not how or why you've gone off the cliff; the fact is that off you've gone and now you're falling whatever distance it is to the bottom.In the case of jumping down for no reason, I agree.
However, there is also the case of falling.
Yes, and both of those are good things.If you make falling damage too deadly, no player is going to want their character to leap off the cliff onto the back of the dragon as it swoops by. Moreover, it makes fighting monsters that have knockback on a cliff extremely deadly.
Trying to leap from a clifftop onto the back of a passing dragon is - and should be - extremely high-risk, with failure meaning death.
And what would be the point of knockback if it couldn't be used to push opponents off cliffs or into quicksand or whatever other deadly terrain feature happens to be nearby?
How does one do this? Dispel Magic won't help against a creature that can fly naturally (e.g. a dragon or a giant owl). A net might, if you can somehow get one up there and it's strong enough to contain the creature. But other than killing it in flight, what other means are there?Lastly, it can be used to trivialize flying monsters by simply knocking them from the sky.
1e has a rule (IMO a very good one) that if a naturally-flying creature is brought down to (1/4?) hit points then it cannot remain aloft and is forced to the ground, but as the creature still controls its descent there's no falling involved.