I'm glad you brought up those videos - I think that's THE BIGGEST reason why North American vehicles have lower payload and tow ratings than EU versions - Safety and Lawyers. The next reason is comfort, which requires softer springs.
Let's be honest with ourselves, does anyone really think a LC70, Defender, or even Unimog loaded to its max GVW can pass a moose test? Should we expect them to? What about a cement mixer truck or tractor trailer?
I doubt even the frame is the limiting factor. There's no industry standard when it comes to frame strength. I'm willing to bet a late-70's "3/4 ton" pickup with C-channel frame is weaker than the modern fully boxed and hydroformed "1/2 ton".
Remember, in the commercial trucking world, your GVW is almost always your GAWRs combined. It's always assumed that your springs are stiff enough to handle the rated axle load. I have never seen "payload" or "tow rating" in a standard Kenworth or Peterbilt spec sheet. Only steer and rear tandem axle load ratings.