Ok. Agree. Everyone for himself would be a possible default (that i'm not in favour of).
A beginner schould, after the tutorial, be able to beat a Numbie. If the Numbies don't team up, more AIs would get even easier to beat. So normally one would want to face AIs that team up.

In Net-games it is the way you suggest (everyone on his own). As this is technically inconsistent, one could argue to make it consistent and thus make it everyone on his own in custom games, too. As so far nobody else complained about it i doubt that it would help make things clearer but would rather make it annoying to have to set teams as it is now in most of the cases you play.

as Bradley said, the default is to have all AI against oneself. The player would be confused if he would find AI 2 ally with him but others to fight him.

I downgraded this bug to a wish and put it this way:

The editor should make clear to the user what the purpose of the team setting actually is.

(the campaign part of the game is in general rather rough around the edges so that's just one of those issues but we definitely should deal with it.)

when creating a custom game, all the AIs are always in the same team (#2), even when the map is defined with them being in different teams.