The problem is though there can be a very fine line between what is deemed "dangerous to society" and what is not. I mean the ones that are beyond help of course you can tell but sometimes it can take decades of regular checking before it becomes apparent someone has slipped that far. Not to mention that no two psychiatrists can ever agree on anything. That is why half the mental illnesses out there have similar diagnoses, symptoms and treatments.
That is leaving out the fact that there is no money to institutionalize them anyways. Hospitals can't afford it, health care isnt going to cover it long term and the government sure as heck cant afford it. Sure if some rich kid splits his personalities his family can afford to send him up upstate somewhere but if the average American Family has a kid go schizo that kid will most likely either end up on the street or living at home being medicated with supervision. There is no room for all of the mentally ill, there is no money...there is no solution.
Plus, if you lock them away, do they have a way to ever get out? Is this a life sentence situation and if it is then dont you need to basically give them every chance to prove they can live in society meaning most of these people will still have the opportunity to do bad things anyways? I mean to prove they are a danger to society dont they have to do something to be deemed as such? It is un-American to just lock people away because they are diagnosed as mentally ill.
Yes I know you said none of that, I am just thinking aloud based on your statements.