> At some point a drug addict, predisposition and circumstances aside, had to make a choice to start taking drugs. Someone with a mental illness never chose to flirt with mental illness, they’re just mentally ill.
as you point out later in your comment, there is significant overlap between drug addiction and other mental illness. it seems unfair to make the "original bad choice" argument in regards to people who didn't start off with a sound mind.
i see a couple other issues with this common indictment of addicts. many people use alcohol without negative consequences, and it is considered acceptable to do so. are you going to go back and fault an alcoholic for taking the risk of having a first beer? another issue (and i believe this may be controversial) is that addiction as a mental disorder may exist in some latent state well before a person ever tries a drug. if so, the question is less "will they become addicted to drugs?" and more "if not drugs, what will they become addicted to instead?".
my final objection is that drug addiction is not particularly unique among mental disorders in that it is possibly caused/triggered by an initial poor choice. explicit decisions (like deciding to try/use drugs) and more general lifestyle are significant risk factors in many mental illnesses (e.g. depression, schizophrenia), but people with these afflictions receive sympathy that is far less conditional.
my personal take: we see addicts behave in ways that appear irrational and antisocial. when we are close to them, it causes us immense emotional pain and it can cost a lot of money to help them. we resent that they seem to be blissfully high while we clean up their messes. there is not a whole lot of incentive to understand how it isn't their fault. the less we can blame them, the harder we find it to draw the line where we can feel okay about cutting them off.