"AP mid" is used to describe a set of champions that typically play in mid lane BECAUSE they scale with AP. That's how it really is.
Let me explain in detail: Mid lane is where you want the champion that is the overall strongest/most versatile on your team. From mid lane you have access to jungle, bot and top. That's the entire map. And that's the reason you typically don't put your ranged AD carry mid: He's weak early and kinda meh midgame, only becomes god in late game. You don't want that kind of champion on the most important position during laning phase. Bruisers are strong early and midgame, so they actually were a good choice mid lane, if only their items didn't suck. And that's why you put your AP champ mid: He's as strong/versatile as a bruiser during the entire laning phase, but scales off of AP, opening up the absolute best item choices in the game.
The major flaw in YOUR train of thought is the following: "The fact that you mainly see AP champions at mid is merely because of the meta." And then you go full stop. You know, there's a reason why meta puts APs mid, and it's the exact thing I described above. APs have by far the best itemization, because they're getting all the love from Riot. And it's gonna continue in S3. Want a proof? Take a look at the new/reworked items. APs came off ahead of everyone else again. Heads and heels above, that is.