Perhaps they are simply implicitly assuming that the threat of being forcibly confined and subject to medical 'treatment' by those who decided that you are broken and need to be 'fixed' constitutes a rather significant punitive deterrent in its own right. People don't usually want to be institutionalised.

The obvious caveat is that voluntary medical treatment for mental health issues must be freely accessible and more pleasant than the forcible kind. Those who wish to proactively prevent themselves from acting out on criminal insanity must have the option of doing so without deliberately committing a crime in order to game the system. Unfortunately those with mental health problems are a notoriously neglected class worldwide. Significant cultural change would be required before this criminal justice policy was coherent.