You obviously dont pay rent, and from what I can tell they also paid for your car and the insurance needed to drive said car. If I were you I would respect their wishes no matter how unrational they may seem. If you think moving out of your house is a better option than living at home for free while abiding by a few stupid rules you are sadly mistaken. You being an 'adult' has absolutely nothing to do with your age, and a whole lot more to do with you acting like one, IE: respecting your elders and their wishes, supporting yourself, etc..
I'd suggest attaining a basic level of literacy before asking to be treated like an adult. Really, it goes a long way. edit - I'm actually not trying to be a dick. If you can't use things like then/than correctly and you don't want to be bothered writing out the word 'you', your parents are probably correct in saying that you shouldn't be smoking pot.
When I turned 18 and my parents tried giving me curfews and holding me down from time to time I just said no and left. Then would talk about it later. Sometimes you just have to be disrespectful to your parents and let them know you're not a baby anymore.