I'm going to postulate that the "beginning" setting played a big role in this. On the lower difficulty levels the AI isn't nearly as agressive, A completely peaceful game with all 9 major civilizations is otherwise neigh impossible. Good and Evil hate eachother too much, and on the higher difficulties, the AI are also quite motivated to try and take a piece of the smallest fish in the pond.
I'm quite sure the non-AI Diplomatic Victory is by design, although I wouldn't have minded if this restriction hadn't been implemented. I think the first one to ally every civilization should be considered alliance leader, and therefore the winner.
However, there is some logic to disabling it for AI players: the AI will never break an alliance to prevent the player from winning a diplomatic victory (otherwise, a diplomatic victory would be very much impossible). The other way around, however, the player doesn't necessarely feel so strongly honour-bound to remain in an alliance if it means losing the game. Because the player can easily "break" the alliance victory of the AI, you might aswell not implement it. (Actually, it would've been interesting if breaking an alliance immediatly led to war, rather than neutrality, with the remaining allies siding on the side of the "betrayed".) I regard a diplomatic victory not so much a player victory, as more of a co-op victory.
