Agreed, driver must be responsible, but only if given accurate weights on the cargo he's hauling.
I'd argue the responsibility must fold back on shipper, if the weight they give to courier is off by some large error (e.g. greater than 10%). Most scales are going to be better than 2% accuracy, so even if they're weighing the load in component parts, the total cumulative error should be well under 10%. I don't think the allowable error ever needs to be anywhere near 300%, which is what your 80k lb. on 20k lb. quote would work out to be.