Because drug abuse is a medical problem, not a criminal one. We should offer treatment, rather than lock up people. Do you know how much jail space we use for non-violent drug users? We let murderers out early to make room for people who never hurt anyone, they just used drugs.
I guess you have to ask, why don't we lock up alcoholics, people who smoke or get high sniffing glue or paint? How about those that eat a lot of fatty foods? Those people are all hurting their own bodies, but nobody else. Same with drug users. But why give treatment to one segment, and arrest others?
Drug users haven't hurt anybody. If they kill someone, or steal property to feed their drug habit, yeah, then you can arrest them. But until that point, it should be treated as the medical problem it is.
Ron Paul clip, in case you haven't seen it. Try really listen to what he's saying. He sums it up pretty well in 30 seconds. =)
http://www.youtube.com/watch?v=88REf0tjZHo