It costs you, through taxes, and the government gets bigger and more powerful and richer off you with the excuse that they are fighting an herb. Guns and swat teams used to raid homes are used against YOU for other reasons as the government sees fit. Wars (even "the war on drugs") are constructed for profit and control.
Consider the idea that there are people who think that they own you and thus think of you as cattle, and they make the laws by the way... it is much easier to understand that they don't give a s about your health, just that you are profitable to them. The criminals you should be most worried about are at the very top, not the bottom feeding drug dealers or dime bag recreational or medical smokers.
Most people I know who have gotten high veg out, become "slackers" for lack of a better word, get the munchies, and are a heck of a lot less problem-starting than drunks. Alcoholics tend to get angry/depressed, potheads tend to get peaceful and paranoid, rebellious (against authority) when they are high. Paranoia creates more questioning such as questioning authority, alcohol is like religion it pacifies and distracts the masses, just as it did to the native American indians when the white man gave it to them to control them. Alcohol and christianity. And oh yea, disease infested blankets... you think you are inherently better treated than American Indians?
Personally I think Americans could use a bit of paranoia because they are for the most part brainwashed out of their freaking minds.