Forcing one to conform to either MT or AT is just not good. Why should I be forced to go one way or another just so I can drive a specific car? It's a friggin' game! Who cares if it isn't EXACTLY right. They might as well make every car from before 1970 leak oil all over the place, and make it to where any one of your cars may just die halfway through a race due to some sort of failure. Realism is great, but too much so can be as bad as not enough.
Also, hasn't anyone noticed that Auto in the game is actually a manual one? Just watch the driver in your convertible during a time trial, and you can see when the game shifts for you in AT, the driver reaches for the stick and shifts it. So, it's not really an automatic transmission. In reality, it is an automated manual. The computer just takes over for the transmission, that's all.