Most adults don't act the way they should when it comes to drugs, their addictions come from the time when they were teenagers and were experimenting or when they were trying to fit in. People do drugs whether or not they are legalized.
The US has two legal drugs on the market. Alchohol and Tobacco. Look at all the problems we have with those. Can you imagine what it would be like if we had every drug legalized? If that ever happens we'll never be able to go back to the first two, we already tried back in the fourties-I think-and that's when organized crime sprung up and wreaked havoc upon everything in the cities. Some of those mobs still exist today. You just can't do it. The US would also look weak in the eyes of other nations if it gave up on the drug war and legalized everything. They're dangerous substances, far worse in most cases than alchohol or tobacco.
You can't have too much faith in the population as a whole.