Hmm, I must be blind, because I don't see how this would purposefully be left out of the game, but here's my issue:

I don't have an XBL Gold account so I just content myself with playing against the AI, but for some reason, when I go to Single Player and choose Quick Match, I can never choose the team I want to play as (or against).

The challenges are OK, but sometimes you just want to have a regular match an AI controlled opponent.

So how do I do it?

Go to Multiplayer->Local Battle, from there you can set up a match between yourself and selected AI enemies.

A bit odd that you have to go via the multiplayer menu to set up a customized AI battle but at least they didn't remove that feature.

quick battle has always worked that way. it is a game with all default settings and random teams. you aren't meant to be able to chose any options, hence, the "quick" part.