I think that mentality is a little bit part of the problem. It's the fear (and sometimes rightfully so) that they will be used for nefarious means rather than being .. what they are meant to be.
Korea and Japan have drug problems as well, but it seems that's not necessarily shaping public restrooms.
Then maybe we need to be creating safe and clean and hygenic places to shoot up where people have access to clean needles?
I ride BART when I visit San Francisco and would NEVER think about entering the restrooms! There are enough "characters" hanging out at the Powell Street station that I tend to keep myself in public, populated areas. Are my impressions correct?
I didn't think BART had any restrooms, at least in SF. In the Market Subway, they send you to the Westfield Mall at Powell.
They have them in the underground stations, but my understanding is that they have all been closed since 9/11.
The Market Street restrooms have been closed for years. But in general your observations are correct-BART restrooms are really nasty. See also BART elevators.