How is this a net improvement in holding only patients who are actually insane?
A patient who believes they are Samson inaccurately believes they have a weakness: their hair being cut. By cutting their hair, you trigger their imaginary weakness, which decreases the amount that they resist, and thus you do not have to pin them down with orderlies.
On the other hand, a patient who believes they are Samson may resist having their hair cut quite forcefully, at least if you do it while they're conscious. A non-superpowered individual struggling to prevent someone from wielding scissors about their head may be quite a bit more of a liability than a non-superpowered individual struggling against facilities built to contain crazy people.
Rationality quotes time!
The usual rules: