Why should a person not be allowed to willingly destroy themselves with drugs? If it is their desire to do so, who are you to get in their way? What if it was something like wanting to end their own life--would you similarly get inbetween them and those desires?
It's funny to me that you don't like it if the government imposes certain things upon people, while at the same time you're okay with imposing upon others your own value judgements with what they should be doing with their own lives.
The idea here is for everyone's benefit in mind. When it comes to drug abuse or suicide, I feel that it is our duty to help our fellow human stay alive.
Let's take a hypothetical situation. A person who is thinking of ending his life may talk to one or two close people to him about it (assuming he has close friends/family). Just by doing that, he's sort of setting himself up for some sort of intervention. You can be sure that his friends will want to snap him out of it and if they want to call for help against this person's suicide, they should be able to. If someone
really did want to kill themselves, they would do it with as much discretion as possible. That way they don't have to deal with any problems.
So yes. If the ultimate goal is to save human lives that are directly being endangered, I would say the breach of privacy is a small price to pay. Although I would never condone supervision in one's home for the sake of security. Only when a problem has rised can law enforcement make a move.