I looked into this a while back (~6 years ago) and it basically broke down like this: diabetics outnumber injectable drug users two to one but not all diabetics use injectable insulin and the ones that do are more likely to do so at home. Drug users, however, have less compunction about sticking themselves in a public space, particular the homeless. Best guess based on these numbers is that it's a 50-50 split between these populations as to who's using public sharps containers.
> society's tacit approval to do what they're doing, thus accelerating the societal rot.
This argument could be made about providing drugs themselves but fails under the faintest scrutiny for sharps containers because said containers have wide uses beyond shooting up heroin.
Homeless heroin addicts who inject themselves in public are not uniformly distributed. Thus while in some places you would expect some such injectable drug use in a public toilet (though I doubt most would bother to go find a toilet for that anyway), that is unlikely for the majority of toilets in a country that contain syringe disposers. On the other hand, one would expect that diabetics who inject insulin would be closer to be uniformly distributed (conditional to the population distribution).
If one could find syringe disposers only in areas where public heroin use commonly takes place that would be believable, but it is not the case.
In contrast, there are actual, real programs and places in many countries to manage issues arising with syringe use, trying to reduce syringe sharing to reduce blood-transmissible diseases etc.
> not all diabetics use injectable insulin and the ones that do are more likely to do so at home.
I'd love to hear the citation for this one, as people using injectable insulin generally need to do in conjunction with eating, and I can't think of where you'd get the idea that T1 diabetics who use injectable insulin avoid eating outside the home as a general rule.
Granted, nowadays plenty of people are using insulin pumps which decimates disposable syringe use, but I'm really puzzled where you picked up that in-home/away-from-home impression from.
Anyone with type 1 diabetes is injecting while outside the home. Usually it's a long acting baseline insulin that you can inject at home and then a faster acting one that you inject before eating or drinking anything that's not water.