I thought the point of the game was to win. It makes absolutely no sense to completely ignore the players that are most likely going to win before you (or even help them win by giving them tribute) and instead waste your resources on warring players that pose no threat to you. Even if the AI is the strongest player, they should be focusing on eliminating their strongest competitors first.
It isn't quite that simple. Taking out the weakest opponent is often the best strategy, and in fact is what a lot of human players do I would guess. Defeat the weakest, take their towns, making you stronger, then repeat until you can beat the strongest. (unless of course the strongest is already looking on the verge of winning in which case as you say it is gang up time)
Not sure if the AI should be following this strategy though, it only works if you actually CAN take out the weakest, the AI seems pretty poor at finishing off weaker AI (let along the player). However if they can fix it so that the AI can conquer their weaker neighbours then that would be awesome, it would allow for more challenging games where the player builds up and then has to deal with the biggest AI who has also been assimilating towns.