Abaddon, what a coincidence! Right now I am reading a book entitled "Why Our Drugs Laws have Failed and What We Can Do About It." It was written by a former US judge named James P. Gray. It gives a good history about how the US ended up with the stupid and insane drug laws that it has.
The reason we continue to have these laws are from what I have read so far:
1) If anyone suggests that the US do anything different than the 'zero tolerance' that it has now, then they are accused of wanting to legalize drugs and giving to to children.
2) Politians won't do it because they are scared of being 'soft' on drugs
3) Law inforcement and government agencies get lots of money through tax money and money seized from drug busts and they would oppose any change to the current system because their money flow would end.
4) People mentality in the US is that pot and cocaine is bad, but other drugs like alcohol and tobacco.
I believe that one day the US will eventually soften it laws. In states like California, Nevada, Arizona and others have passed, by large margins, laws softening current drugs laws. However, they have always been shot down. I believe that the pressure will increase and we will have drug laws that will show some sanity.