Why is marijuana illegal in the United States?

its illegal for several reasons. 1 is there are many misconceptions about what the plant really does to you.any smoker can tell you that its side effects are minimal at best. far less than alcohol. 2 is that it would be very hard to tax. its so easy to grow. some may say tomato's are easy to grow and onions are easy to grow but people still buy onions and tomato's at the supermarket. the problem is that 3 tomato's plants will not produce a years worth of fruit. 3 marijuana plants will produce more than enough smoke for one person to last all year. and that's for a heavy smoker!. I have quit smoking weed. it only real side effect is that it robs you of motivation. I'm returning to college so I need all the motivation I can get. however I still support legalizing marijuana. its not a crime at all.