There are some things that the government needs to get out of. If someone wants to smoke a joint, fine, no problem. Tax it like they do other "sins" and leave people alone. If something is causing no harm to others the government needs to ignore it.
If you have 30 people and a keg in one room, and 30 people and 2 or three pipes going around in another room, which room will you needs the police in?
The US needs to end all attempts at being a mommy state, and start letting people take responsibility for their own choices. The Constitution was written to limit government intrusion in people's lives, and we have gone far, far afield from that idea.