A law of society is not about preventing people doing things they shouldn't. It is there to encourage people not to do it in the first place.

Does the death penalty prevent murders? No, of course not. It does (theoretically, I don't believe in the practice myself) try and discourage violent crime. Introducing a law (prohibition, gun control etc) would surely take some time to encourage people to stop what they are doing. A sort of time-delay. Just because politics rarely takes a long term veiw, doesn't mean that attitudes can't be changed.

It's all about limits. Alcohol and tobacco are dangerous drugs that kill many, many people each year (eventually). However, they are tolerated because they are the limit of acceptability. If heroin was legal, many more people might die from overdoses - although some might argue that less would die from anthrax :-P

A cruel point of veiw would say that harsh laws make a few innocent suffer (as well as many guilty) for the sake of society. Alternatively, the few guilty who escape justice are worth it for the innocents. Who knows?