It just seems like both drugs have similar side effects to them. Why is one legal and the other not?
Follow the green paper trail. Alcohol generates too money to be illegal and there would literally be blood in the streets if you tried to take it away from people. Marijuana will never be legalized because it would cost too many people too much money. Plus, you have several generations of people that have been brainwashed into thinking it "really" is somehow worse than alcohol. Medically speaking, alcohol is clearly worse for you. Nicotine, the other most popular legal recreational drug, is even worse than that. Believe me, the illegality of marijuana is a complete double standard. We don't care if you drink in this society or consume tobacco until you can't breathe anymore. But if marijuana is your drug of choice the same guys that drink themselves into a drunken stupor every weekend will be making jokes about you for the rest of your life.
Speeding is illegal and 90% of us do it to some extent anyway. Just because a law is a law doesn't make it fair. Look at the Jim Crow south, for instance. Ricky's mistake (in the past, but not necessarily this time) is not being able control his urges against the risk of losing his career.