It is an opportunity to say "Ok, we can't stop these people from doing heroin. But we can make it as safe as possible, and we can provide a vector for additional health care interventions". It also means that you aren't psychologically torturing addicts by telling them that if they were only 'strong enough' they could cure their addiction.

You might not like the way HAES or fat acceptance is packaged, but this is exactly the same thing.