If the AI wants the city with its massive stacks then it takes it and we get to take advantage. The AI sucks strategically. If it didn't it wouldn't need ridiculous production bonuses. He isn't worldbuildering or otherwise breaking game rules, rendering it a perfectly legit move.
I second that.
The AI has it's bonuses, we tactics like this to dezimate huge stacks, I see no exploit.
It's similar to the old AW-tactic in Civ3 of "baiting" the AI with an empty city to a favorable "killing zone".