It's a natural instinct to tighten these up very firmly. I was stunned how easy it was to dislodge the thread on a 5D grip. The small metal insert actually broke away from the body. And I'm definitely not Mr Muscles. It was a $110 fix. Could be that these threads were never intended for the stresses that a BR or similar system will impose.
I'd phrase it differently - what I think you were stunned by is the amount of force that a screw mechanism can generate. A basic review of physics - the force generated by turning a screw is much greater than direct pressure. A screw converts a longer rotational motion into a short linear motion, effectively magnifying the force. That's especially true when you're using a wrench/allen key as you'd use with the SpiderPro system. Consider the
apparently little force you'd need to screw a large lag bolt into a wooden beam with a socket wrench vs. driving a spike into that beam with a hammer.
I think the tripod socket is designed to withstand the stress of the weight of the camera/lens, including the additional centripetal force from that weight swinging at the bottom of a strap. I've logged many hours of swinging around a camera + lens combo weighing over 5.25 lbs/2.4 kg (gripped 5DII with 85L II) supported by the tripod socket.
Now...how about screwing in that 1/4"-20 bolt? I'm not Mr. Muscles either, but let's say I pull with 20 pounds of force (try it on a scale, it's not going to be nearly as hard as you can pull). Assuming a 3" allen key, that's on the order of 37 lb-in of torque, which translates to a clamp force of 740 lbs. About half of that force is lost to friction during the tightening, so let's say when screwing in the bolt, you're applying the equivalent of about
370 lbs/168 kg of weight on the threads. So, while you might be stunned by
that number, the idea that that amount of weight might damage something on your camera is probably not too surprising.
The short version:
don't overtighten a bolt!
