If the objective was to help addict we would help them instead of throwing them in prisons.
Furthermore, alcohol is not forbidden because some people are addict.
Ultimately, I would add that drug abuse by itself is not the problem. The problem is that some people are self destructive. drugs / alcohol is simply one of the ways to do it. We should help them.
We can't say all the harm from drug use is because of the illegality. Look at the carnage caused by alcohol. (At least, in the UK.)