I do and don't understand that perspective. I understand wanting a justification, but it's always seemed fitting to me. Many AoE spells/effects originate at a point and spread outward. Fireball is probably the most obvious example, but I tend to think of most effects that way. You're a super smarty, so as you extend the AoE outward, you are able to selectively shape it at the margins, where it terminates. You can't do it on the interior because that's the origin of the effect, where it's emanating from. That's always been my view of it, anyway.
I really don't like this idea (having the outward portion of AoE spells be foes only). It feels very game-y. Sure it helps balance but at the cost of consistency and elegance.
Moreover, you remove the AoE trade off (I hit a bunch of ennemies but my frontline too) as people will position their spells to only hit with the outward ring. This will lead to odd situations where you throw your fireball in the middle of nowhere to maximize the outward ring...
As for the "RP explanation": anything can be explained with RP but it still hurts the suspension of disbelief.
This being said, props to Sensuki and Matt for their work, and to Josh for his participation