1) It encourages other forms of drug use, the idea is that if you use marijuana you're going to be tempted to use harder drugs. This is like saying that if you drink beer once or twice a week you're going to be a wino alcoholic by the end of the month..
2) Marijuana is not a "socially accepted" drug, that is it is seen as a substance that encourages immoral behaviour and criminal behaviour.. unlike alcohol which is well know as a substance that encourages polite civil behaviour amongst our population
One more for the issue, 3) It is seen as an addictive substance which will cause youth to engage in criminal activity to feed their habit... unlike tobacco and alcohol of course..
Yeah, I'm kinda pro weed if you hadn't figured it out :)