So you’re proposing we turn addicts into slaves. Interesting.
The fundamental reason I suggested that is humans, particularly the type who suffer from mental illness or lack the ability to care for themselves, to the extent they end up destitute and addicted living in filth and on the sidewalk, require some sort of structure imposed on them. In the past that was done by the family, in our current world many of these family bonds are, sadly, falling apart.
Putting people in an asylum, only to sit around on a bed all day and take copious amounts of anti-psychotics is a miserable existence. Having the opportunity, if one chooses, to engage in meaningful work, can provide structure and meaning to life.
I'd prefer to think of it more as a therapeutic sort of work program, where those under conservatorship by the state learn how to live with structure, learn what it means to make something, and could even be compensated for their efforts.
In my mind at least, I am contrasting the reward of a hard days work tending to, say, an organic vegetable garden, with the mundane horror of sitting on a bed in between distributions of anti-psychotic medicine.
I hope my clarification gives you more context to realize I'm not suggesting slavery.