Simple logic

Prison = bad.
Prison + mental illness = very very bad.

Get it?

"States can institute policies to keep mentally ill inmates out of segregation or wait for litigation to force them to do so. They can improve the quantity and quality of mental health services they provide. But even those necessary steps fail to confront the larger question - should prisons be de facto mental institutions? If not, then the country must reduce the number of mentally ill people who are incarcerated. "