Based on my experience it's definitely force. I had a kerbal step out of a capsule and into a chair on a small rocket before launch. He made it the whole flight but it was a short flight without a lot of delta v. On a separate trip I flipped a Mun rover off the ground, my kerbal landed some meters from it alive. I think if there were stronger seat belts he might have snapped his neck. It's a side effect of believable physics as far as I can tell. side note- when I heard about the external chairs the first thing I thought of was motorcycles, the chairs imply risk.