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.