Why not? Folks who are being held after being restrained by police are left alone in holding cells all the time. Sounds like they were letting her try to cool off a bit, after her outburst in the gate area.
This is such a sad story. The family bears a large part of the responsibility. They should never have let this woman travel alone. Since she was on her way to a facility for alcoholics, she was in a tenuous state to start.
They left her alone HANDCUFFED. Your friendly neighborhood jail doesn’t even do that ~ not even to the nastiest thugs and curmudgeons.