the law against being stoned is called PI or DUI if you are operatong a vhicle. States have passed amendments that include any intoxicants, which include pot.
There's a law against driving while asleep, that does not automatically make sleeping a crime. There is no law against being stoned, because that's too close to thought crimes for our legislatures to stomach while we are still pretending to have a Constitutionally limited republic, affirms sacrosanct individual rights, rather than a communist nanny state.
...and by the way, extending DUI to marijuana is unscientific malarky. The NTSB, and the Calif. Hwy Patrol both ran extensive controlled experiments on stoned drivers to try to metricize the blood stream level that could be deemed DUI. No such levels were found. No matter how stoned the stoned drivers got, they still outperformed their control counterparts on a large percentage of the tests. Turns out that while marijuana produces a slight temporary reduction of short-term memory, it compensates by making users more attentive to the moment, and memory isn't as useful to drivers as attentiveness.
If you'd like to test this report, try asking a cop what the exact level of cannabanoids in the blood stream is that consititues DUI--the answer will be: ANY. Pretty remarkable, eh, given that they did extensive testing to establish exactly that. Just another example of what brazen, out-and-out thugs the drug warriors are.