your parents are over protective for being mad at you for smoking weed if your 18. unless you clearly are abusing it (pretty hard to do with weed) and its affecting your life very negatively (also hard to do) then they should not even be thinking about rehab. In my opinion, smoking or not smoking weed is a personal choice that shouldn't be forced on anyone.
however, your parents are damn right to be mad at you for being a drug dealer.