I didn't think you could have seat belts not attached to the primary structure?
Sure you could. Lots of planes have seat belts attach to the seat frame, not the airframe. Most GA aircraft have seats which move within the airframe, or are removable, and thus might not be connected to the primary structure with the integrity one might expect. Having the seat belt attach to the seat rather than the airframe simplifies dealing with changing seat belt lengths with the seat moving.
In nearly all of these aircraft, those seats are only approved to hold safe a 170 pound occupant. The seat belt might hold a greater load, but if the seat structure (plus having to carry the loads of its own weight too) won't carry the loads that the seat belt imposes upon it, there's not much point!
One of my client's aircraft, involved in an accident, had seats break away from the floor, with the occupants still belted in. These are seats which I refused to load test, when such test was required for approval in that aircraft. I'm glad that I refused to test or approve these seats.
I have a Cessna 172 seat belt attachment fitting, which was bolted directly to the floor. It was pulled right out through the floor during the crash. The seats of that accident aircraft also broke away from the floor. The seats would have broken away a lot sooner if the seat belt loads went into them, rather than directly into the floor.
Our last line of personal defense against injury is remaining in place in the aircraft during the crash. Seat belts are the only way that is going to happen. Neither seat belts, nor seats were designed for occupants whose weight exceeds 170 pounds, except for some specific special purpose aircraft. Those occupants who exceed 170 pound (including me) have to remind themselves that their safety is reduced proportionately to the amount of their weight which exceeds 170 pounds, as that is all that the plane is designed for and certified to. So best to mitigate that risk by not allowing a weakened or damaged seat frame become a part of the path of load capacity for personal security. Belt to the primary structure, and bypass the seat....
This is only a bad idea when the seat belt is attached to primary structure which might be torn right away with the belts attached, during the crash. The old arrangement of attaching the back seat lap belts to the aft float fittings through the fuselage skin comes to mind. If the floats are torn off entirely the seat belts could go with them, and that would be very bad for the occupant!