Because it has been shown, time and again, to enable the addicted to ween themselves more easily than going cold turkey.

what do non-humans believe?

It might change the routine they go through, what with the new effects, methods to obtain said new addiction material and all, making their life seem slightly different. A lot of addictions seem to fill a void presented by emotional turmoil, lack of self value and all that, therefore they're always strongly associated with one's life, memories and feelings; switching from heroin to drinking for example might certainly not be voluntary, and aside from the biological factors that come into play when it comes to physically needing something and mentally needing to ''depend'' on something depending ON the addiction and what it physically does, it might give these people a whole new perception, outlook or huge change which makes them think it's not an addiction, or at least, is different enough for them to be something else entirely. An addiction is part of one's life in so many ways, and obviously, usually the core OF their life, and having lived with it a long time, they might not be able to break away from whatever fundamentals have rooted themselves in their behaviours and day to day process, even if they no longer physically need the substance. As well, morality and values imprint themselves in one's mind to justify the addiction, and can be hard to break away from, and this conditioning needs be catered to.

Last I checked, I am human and I dont believe that. There really is no cure for addiction, just the ability to control it.