The AI seems to be pretty dense when it comes to creating colonies. Even on very high intelligence levels, it doesn't seem like it is smart enough to consider the impact of Opponent's Influence on a new colony. Just some examples from recent games:
In my current game, my Influence is the second highest (2nd only to the Thallan), and it's good enough to push against them to keep their border from enroaching on my claimed worlds. The silly bugs recently sent a colony ship all the way to the heart of my empire to colonize Mars.... Obviously, they had no chance of holding it, it flipped to me before they got the 2nd embassy up and running on it.
In my last game, by the latter part of Mid-Game I had a sprawling Empire that crossed the map at a diagnal. My sphere of influence at the ends was large, but in the middle, not so much (Sorta shaped like a diagnal hourglass), because the Krynn and Thallan were on each side of me in the middle pushing back relatively hard. The Drengin, crazy little guys that they are, kept colonizing world after world down in my southern end. Some of these flipped to me so fast I hadn't even noticed they were settled when I was getting the message that the planet chose to join me. AT one point when this happened, I was actually at war with the Drengin, so it HAD to flip fast for me to not have noticed a planet with their flag show up in space I "owned". I got about 7 or 8 colonies this way in relatively short order (after which there were no more non-settled planets in my territory).
Considering the investment of both construction time AND population that Colony Ships require, it seems a bit crazy for the AI to basically just give the player free colonies. It shouldn't be too hard for the AI to do an Influence check on the environs of an uninhabited world and decide "not gonna work".
This has always seemed a problem to me, but I'm only saying something now because it's only recently I've played with difficulty levels where I didn't just assume it was the AI intentially being stupid. Set above average, the AI should NEVER do this, imho. And it does it, pretty much always. I could intentionally hamper nearby opponent economies just by leaving many low class worlds unsettled in the heart of my empire.