If that were all they were doing, and staying inside, that'd be one thing - but it generally ends up going far beyond this, at least here in the United States.
Such people through those actions, of using major hard drugs, are generally at a point where they don't care what happens to them, or their bodies. They then eventually go out in the public; when they aren't bothering anybody, their antics can be entertaining at best, and downright tragic at worst.
When they do go to bother somebody - and they will, and do - they generally won't leave you alone. In some situations, they can get extremely violent. Also, regardless of whether they are violent or not, depending on their level of usage and other factors, they may literally look - and sometimes are - "the walking dead". Coughing, spitting, seeping and oozing open sores - this and more can be found among these poor souls. One hopes they can find help, but more often than not they refuse it when offered (some will even claim to be healthy and sound, despite the reality).
Eventually some turn to random violence or other crime, quite often breaking and entering locked vehicles and homes, stealing whatever isn't nailed down (and quite often stuff that seemingly is - not even rooftop mounted air conditioning units are safe).
I'm normally all for "freedom of choice" - that one's body is sacrosanct and should be up to the owner what can and should be done with it. But some of these drugs have shown that, when abused, they can lead to worse outcomes for the public at large. Part of it does have to do with the illegality of the drug, and it's availability. Maybe if such drugs were legal and free, and shelters or other places available to use of the drugs (with appropriate medical or other staff to ensure abuse doesn't occur, or that people can get help or treatment as needed or desired) - such outcomes would be different or non-existent.
As it is here in the United States, though, at a certain point users of these drugs become a problem for society and its safety at large; they should have freedom - but not the kind of freedom that allows harm to others.
Also, addiction, and addiction, more than anything, is related to trauma.
If someone constantly rented the car at night, didn't drive it, and was dodgy whenever they came to a lot, I guess you could say it's none of our business except that it's a $10,000 insurance liability along with a branding issue if someone is say doing hard drugs in our cars while people are walking by.
These quite rare investigations only happened after finding things. Sometimes cars would be out for 10+ rentals before arriving back for a cleaning. The frequent night renters would be on the previous renter lists multiple times after finding things so it was more to clear their name from suspicion than anything.
Honestly I don't really have a judgment on someone say, freebasing meth, that's their freedom. Just please not in our cars.