Thanks for the comments rapaleeman and H4nd0fg0d.
On the question of whether the AI are actually programmed to be pushy, or are just stupidly staying on their line, I'm certain that in many cases they are definitely programmed to be pushy. There are some cases where an AI runs into my side in a corner, or tries to get back on their line if I've pushed them, where they may just be following their line and not going around me. But it's very common that I'm just passing an AI car on a straight, or they're passing me, and they swerve over half a track width just to bash me as they go by. So I'm 100% certain they're intentionally programmed to do that.
As to whether it's fun to have the AI behave that way, that's obviously subjective. I find it fun some of the time, but especially as the races get tighter, I just end up wishing it was a more pure racing experience. I do like car combat games. I've spent many happy hours playing Split/Second and Sonic All-Stars Racing Transformed on my Xbox, and Asphalt on iOS. However I tend to take those games more lightly. Sure they take skill, but I go into them knowing that they're going to be very chaotic, and luck will play a big factor, and I just have fun with them. In other racing games like Forza 4 or F1 2011, I get much more focused on refining my driving and racing skills, and I want the game to reward precision and finesse more. I guess I'm just saying I like my driving games to be either full-on combat/stunt silliness, or more focused racing, and Slipstream seems to occupy some in between point that bugs me sometimes. Overall though I enjoy it, and the driving itself is so fun that it keeps me coming back.