Resident psychiatrist here. That is simply not true. [See similar comment below]. I'm a resident psychiatrist, almost all patients we admit at my institution sign in voluntarily. The times we involuntarily admit patients are when they are so symptomatic that they would very likely die without treatment (e.g. they refuse to eat because they think their food is poisoned).

Psychiatrists have no interest in detaining people, quite the opposite. Most of the time I'm dealing with patients who are feigning symptoms e.g. pretending to be suicidal to get admitted because they're homeless, and so we have to admit them lest we get sued, despite the fact there is good evidence many times an admission would be counter-productive. You have people who will sue psychiatrists if they don't admit someone and that person ends up killing themselves, and you have people who think psychiatrists are wantonly admitting too many people.

Also there's nothing dubiously ethical about ECT. It's one of the safest procedures in psychiatrist and the most effective. Many many patients consent to do ECT voluntarily.