LOL
I must say the scorched earth thing is really really annoying (gotta rush build a colony ship to offload population, which gets repetitive), and often counterproductive for the AI. It destroys its own cultural resistance buildings for example. I have presumed that the point of this behavior is to diminish the value of the worlds that the human will soon gain.
The problem with this is that the AI is effectively conceding victory at this point. Since it's not building any cultural starbases of its own, you can just take over the galaxy sector by sector this way. The AI rarely if ever does anything to actually stop the human from taking over the galaxy by building Party Palaces. Sure the worlds the human takes may be ruined, but the human still wins.
The AI is smart enough to threaten war if you build a shopping district in its sector, but not smart enough to recognize that eight Party Palaces in the adjacent sector is a threat--even if they're in sensor range. Or sometimes it's smart enough to destroy its own worlds to diminish the reward for the human, but for some reason it won't threaten war or demand the starbases.