We don't put users and addicts in prison. Our laws are designed to put dealers in prison. Okay, sometimes a mere user gets caught with a stash as big as a dealer usually has and gets thrown in the slammer. But such cases are rare.
And no, not all users are addicts, but many users turn into addicts, and I frankly see no point in taking the risk that a mere user will turn into an addict.
...and the constitutional issues?