However, the government have a legitimate right to prevent citizens from doing things that will be fatal to themselves or that might seriously harm other people or property.
If this is your premise, then government has a "right" (I'd love to see where that right is enshrined in Common Law) to monitor your caloric intake (eating too much is harmful), ensure you don't go to McDonald's too much, make sure you eat the right kinds of foods, make certain you don't drink "too much", don't take ANY risks our "good and wise" politicians and bureaucrats deem unnecessary. Your line of reasoning would almost undoubtedly lead to an overarching government that knows no boundaries to its "rights" since it is here to protect us from ourselves.
Also, you act as though government has the ability to keep people from buying and selling drugs, alcohol, or tobacco. I would have thought that the Prohibition Era and Mexican Drug Cartels would have taught prohibitionists like yourself that government is powerless to control such things. You only drive the activity underground, empower criminal organized crime and gangs, and turn people who would otherwise be considered law abiding citizens into criminals.
By the way, how's that War On Drugs going so far? Are drugs any less available to kids today than they were in the 60's when this never ending war began? We've spend hundreds of billions of dollars on this War On Drugs over the past 40 years. I'd like to know when I can expect for us to win.