Their is actual, recent, scientific research proving weed can be bad. If your mind is still developing. Say if your about 16 (ring any bells Kintaro?). And smoke cronicly (I can't spell, damn school system and mull!). It can have some really bad effects, like phsycosis and scisofrenia. But in like 10% of cronic smokers. It's possible but not likely. Besides which I don't think congress has seen any very concrete research. Mainly because there isn't any. There where some studies done when mull was first made illegal but they where buried because they said it was good for you. Keep in mind that they didn't get very far. Wonder why? Bush had nothing to do with this however as he wasn't even born.
And then theres hemp. Which you need to smoke about 3Kg to get high on. It's the strongest natural rope, makes a chipboard thats 10x stronger, great clothing, I could go on but I'm to lazy to research it. Now why is that illagal? You most defanantly cannot get high on it. By the time you do you'll have died of smoke inhalation. That and apparently it tastes horrible.