That's unfair. It isn't right to talk about doing things against the will of an individual who isn't a free actor due to their condition. This is very well established notion legally, culturally and morally. For example, if a schizophrenic person commits murder, their condition is a mitigating factor in their sentencing and will prevent them from simply being locked up with the general population.
Similarly, you're already treating them as individuals incapable living up to the responsibilities of living in society. All municipalities have zoning laws to control where citizens can live and laws against squatting in tent-cities or on a sidewalk and living in squalor and filth, without proper sanitation, and protection from elements - yet somehow you still argue that those laws should not apply those individuals. So you're trying to have your cake and eat it to. On one hand, you're saying forced institutionalization where care could be provided, is wrong because those individuals CHOOSE not to be institutionalized, and on the other hand, you're arguing they shouldn't be held to the same responsibilities as other citizens because they can't help their situation.
This position is arbitrary and incoherent.
Finally. I disagree with your contention that institutionalizing people for the purpose of providing care (not locking them up - as you called it) who aren't capable of making their own decisions is immoral - and you haven't attempted to show why it is, you just assert it as a fact - which it isn't.
>What's wrong with people in the US that this even has to be discussed
I'm not American.