My parents gave me hell when I was in my teens and still living at home. Same thing as you. My smoking, and their disapproval of it, really strained our relationship to the point where I couldn't wait to move out. Now I've got a 3 year old daughter and can completely understand everything they did back then.

Seriously, right or wrong, their hearts are in the right place. They just believe (about weed) what they've been trained to believe for years and years. It may sound cliche', but your parents only want what's best for you. Cut them some slack and try not to let it get to you. Do your best to preserve your relationship with them. 10 years from now you'll be glad you did. I know I am.