Why so ignorant towards marijuana?

I was under the impression that people only get affected so badly by this plant are the ones that get allergic reactions to it. And people who have preexisting mental and health issues should always take serious steps in questioning what ever the heck they put into their systems.... mary jane or otherwise. At the end of the day, people should just leave people be. Smoke pot, go ahead, smoke tobacco? why not? eat chocolate, go right on ahead, drink caffeine well, big deal? We should be adult enough to make these choices for ourselves and screw what ever anyone else thinks.