There is a Driving Mission (can't remember which number it is, offhand...) which requires you to do exactly that. Slipstream as much as possible to lower your overall lap time, so you can pass the final car at the very end and succeed in the mission... So, if there's a mission that's trying to teach that exact skill, why should it be illegal in a normal race?
And the skill involved in slipstreaming is not trivial. It's MUCH harder to stay back there and NOT hit the AI driver than it is to pass him on the first turn, in my opinion... I've left the AI settings at their default for all my races so far, and can easily pass on the first turn, which kind of defeats the purpose of having a "race" instead of a "time trial." I might increase the AI's abilities and give this slipstreaming thing a try -- it'd be nice to be able to pick up an extra 10 to 15 mph down the long straight near the end... But it's not going to be easy to follow the AI for the entire track-length, just to get to that point... Anyone who can do that without screwing up and bumping the AI is an excellent driver, in my opinion, and deserves the faster time...