The drug addict that is being ridden by his/her demons is going to be going after that fix no matter what. The harder it is for them to get their fix, the more damage they do to the world around them.
Addiction is literally a monster on their backs. Addicts are always aware of it, and as an addict that worked in my crew once told me, when it gets too bad, it's like "something has just grabbed into her skull with long red claws and is taking control of big chunks of her brain".
Yes, of course she's going to "just say no" if it's "too hard" to get the drug that eventually calms the monster that's currently ripping her mind apart and making her physically ill. Let's just punish her some more because damn it, she was weak, and let her then-boyfriend convince her to get strung out with him that one time back when she was 16.
Give addicts a safe place, give them access to help to control their addiction, and there's less overall damage done. That's not encouraging them to do drugs, and not going to be getting them even more addicted - the addiction ship has already sailed, and the addict is well and truly out to sea.
They're self-medicating with the addiction as it is. Withholding the drug they crave at that point in time is not going to somehow make them see the light, make them more likely to quit, etc...it's only going to make them more paranoid, sneakier, and more liable to cause themselves and others harm as they desperately seek out their next fix when the urge gets too bad.
Give them a safe place where they can find help, when they're ready. They're still going to hit bottom - just not with so much added danger to the rest of the general public as they start to implode.
Haele