Used to work in the freight department at ET a looooong time ago. Since various incidents -- and especially since that awful fire -- the H+S regs are extremely tight BUT they're there for your safety. I agree that it may not make much sense but I'm sure there's a reason for it. The contents of your load may also have a bearing on their decision.
Would it work out cheaper than freight if you split the load over two vehicles? What if you put some of the load on your passenger seat? Or, what if you ingested some of the load using condoms and took laxatives at your destination?!