Why is marijuana such a big deal with law enforcement?

Ok, now I don’t care much for the stuff but I don’t understand why we waste tax payer money to arrest people for using and distribution of marijuana. I personally think alcohol consumption is much more dangerous then smoking a joint. Alcohol makes some people very violent whereas weed just makes you want to raid the nearest vending machine. I don’t understand why you can get thrown in jail for selling a plant that you can technically grow in a well lit closet. Perhaps some of you can clear this for me why our society wants to utilize precious resources for curbing something that I don’t feel is really a big deal.