Bobby has a good point, what if you accidentally chop someone if half with your C152 prop? If you had smoked a couple of weeks ago there would be traces of MJ in your system and you might test positive for illegal drugs when the NTSB does its accident investigation.
wrongful death?
But, on the other hand the fact that alcohol is "legally" out of system after 12 hours doesnt mean your safe to fly either. Your BAC could measure zero and your sloppy at best. I've flown after a night of partying and well, too say the least, I felt like crap and I didnt need to be anywhere but in the bed. Alcohol has some serious aftermath effects on ones mellon. Most pot smokers I've known swear that they feel no ill effects the next day.