Maybe it is because people with borderline mental illness will be economically and intellectually challenged. They'll be less likely to be in a nice restaurant or to organize their life enough to make a healthy dinner for themselves.
I don't claim this is a major factor but fast food restaurants are more commonly in urban areas where the water supply has little or no lithium.
Lithium is a natural mineral not just a medication given to some violent mentally ill people. Studies suggest that communities with generous amounts of Li in the well water have less violence. http://www.ncbi.nlm.nih.gov/pubmed/1699579#
If a person is struggling to control anger even "justified anger" and the lithium on your hair or blood test is quite low, you might like even just a partial capsule of the 5mg lithium orotate supplements that are available.
I was very pleasantly surprised at the nice, natural "chilled out" effect on my previously intense personality. I'm not suggesting it will be a fix-all but possibly a nice puzzle piece for you.