I genuinely don't know much about this question. What makes them more capricious than DUI limits for alcohol for example? People vary in their metabolism and intoxication levels there - is there much greater or less predictable variance with marijuana?
I also have to admit that trying to in effect argue that people should not be prevented from "driving stoned" (yes, I know that's not really what they're saying - I hope!) is a peculiar strategy to get the majority of people onside. I rather like a drink. But I wouldn't argue in favour of drink driving.
"What makes them more capricious than DUI limits for alcohol for example?"
Because if you look at the NHTSA data, it's very clear that the risk for someone driving at .08 is about 5x the baseline risk. If there was similar research showing that driving at 5ng/ml of marijuana had the same 5x increased risk then it would make a lot of sense. But that is not in fact the case. What they are essentially saying is that it's ok to drink and drive until your risk of killing someone is 5x increased, but it's illegal to smoke and drive even if your risk factor is only 1.1x or whatever.
I also have to admit that trying to in effect argue that people should not be prevented from "driving stoned" (yes, I know that's not really what they're saying - I hope!) is a peculiar strategy to get the majority of people onside. I rather like a drink. But I wouldn't argue in favour of drink driving.