CanadianJeff CanadianJeff:
It's a valid question but 10 seconds of thinking and I came to a simple answer.
a) if you did drive under the influence of those drugs and the police found out you would still be charged but...
b) Very few people are on those drugs compared to how many people would drive stoned if pot was legal so...
Really? Are you certain? Do you have any information whatsoever to back this up? I don't have information on how many people drive while under the influence of prescription or over-the-counter drugs, but my hypothesis is that there are probably more people driving under the influence of, say, dextromethorphan, which is the active ingredient in cough syrup which serves no real purpose other than to get you high, than there are people driving under the influence of THC. These packages that say "non-drowsy" on them might give drivers the confidence to drive while they're taking those drugs, without realising that, despite not putting you to sleep, may still impair or slow your judgment.
$1:
c) having a lot of people on a substance that slightly impairs driving could cause more risk for normal drivers then simply having one or two people hopped on on harder stuff on the road.
It is quite possible that there are already a lot of people driving around on substances that are not THC which impair motor skills and reaction time.