I agree with you, but it's worth spelling out exactly why. Philosophically, prison has one of three purposes: protecting society, rehabilitation, or punishment.
If the sentence for use is anything less than spending the rest of your life in jail, then society isn't really protected from addicts; the recidivist rate is too high. Get out, steal again for the next high, rinse and repeat through the revolving door.
Prison is not an environment capable of rehabilitating addicts. Leaving aside drug smuggling into prisons, getting people past the withdrawal period is the "easy" part of rehab, the real work is in therapy, developing better relationship models, better boundaries, better strategies for the triggers that drove people to look for a high. It's very, very difficult to facilitate that in prison.
Finally, it's debatable whether prison is actually punishment for addicts. If they keep access to drugs through the people who smuggle them into prisons, and the prison pays for meals and a roof that were often lacking for the addict on the street, then, is it really a punishment?