I agree, but the same applies to many other prescription drugs that are not specifically addressed by 'under the influence' laws. Driving and smoking tobacco is obviously legal, and nobody seems to worry about that. Obviously there is a continuum here. Driving tired can cause just as much impairment as driving drunk. At the end of the day, people need to be responsible for their behavior. I don't see a reason why MJ should be specifically addressed, just because it was illegal yesterday.
Suppose that someone is naive enough to think that since MJ was previously illegal, nobody ever drove while high. Thus, we don't know what MJ's impact on driving impairment is. So, does that mean we should make it specifically illegal just to be safe, or should be make it legal, just to be on the side of liberty?
Besides, people drive while high all the time now. I don't think the number of people smoking and driving is going to go up. If anything, it might go down, because a lot of teenagers seem to smoke in their cars.