> most people go through some vulnerable time when they could become addicted (or otherwise abuse drugs), and at that time you don't want to have the stuff in your house or a dealer friend ready to supply

Sometimes people do drugs to cope with their other problems in life. They need drugs as a reprieve. Life can be hard and people have used alcohol, pot and many other substances to create a refuge. Of course that doesn't solve the problem, but solving the problem is not always the top priority, and many times the problem has no solution. Why should we morally deny a refuge from anxiety and suffering to other people?