Before commenting on needle exchanges, I want to question this line.
I contend that the only way a drug user has ever stopped using is either though their own choice, or imprisonment.
Do you have any evidence for this? If you mean imprisonment by the government then you're definitely wrong. People go into withdrawal against their choice for a variety of reasons that don't involve the government locking them up:
Addiction is a tremendously difficult disease to manage and some people only stop using when their loved ones strong-arm them into accepting treatment. Once they get the treatment, they can stay clean by their own choice, but the initial efforts weren't voluntary.
Of course, you can contest the morality of forcing someone to accept drug rehabilitation. But it certainly happens, so
Personal choice, although difficult, is the only way to get a drug user to live a clean lifestyle
is not really true. Personal choice is often the most important contributing factor, but other factors like a support network make it much more likely to eventually become clean.
I say all this to point out that insisting that people choose to get better is a great way to introduce inefficiencies into your solution. Personal choice is, basically by definition, the property of a person others are least responsible for. By requiring drug addicts to choose to come clean, you are limiting your possible solutions to only those you have no responsibility or control over. You're basically saying, "drug addiction shouldn't be anyone but the addict's problem". You may personally feel that way, but don't pretend for a second that this is an efficient solution.
If interventions increase the chance of an addict going to rehab, and going to rehab increases their chances of living a clean lifestyle, then you, as a policy maker, should support them. If needle exchanges make recovery more likely, then you should support them. If they don't make recovery more likely, then criticize them on those grounds.