[BeHoldAPaleHorse]

Are the Marines expecting one of these guys to try to bolt or something?

Two things come to mind:

1. Risk of retaliation against a witness;

2. Suicide risk.

[RetiredArmy]

Witnesses are not in the confinement facility. And, if under suicide watch, which I know of some who were, a guard merely stood at the door and kept eye contact on the prisoner. No chains involved.