As the title states, I encountered this problem every game. I even tested it by allowing the AI to take cities, a few times they razed it immediatly.
From my own experiences you cannot Raze a city you captured until you've owned it for 4 turns. So the AI should have this rule as well I'd figure.
Note: I was playing on Easy Difficulty, I also tested it on Normal, the Bug seems to be for every level of difficulty.
