Don't forget there are a plethora of 520d's keeping the price of the '5' policy down. All of the 7s are 6, 8 or 12 cylinders with much greater levels of standard equipment. If you think a 330d cylinder head is expensive, try fixing the V12 in the 760..
I take your point with the V12 and V8 engines in the 7 series for sure. However, the 730d engine is pretty much the same as in the 530 as far as I can make out, a six pot that's fitted to loads of 5's and X's. I can't think of a technical or engineering reason why the warranty is so much more for the 7. Me thinks it's a BMW marketing reason and retaining the exclusivity of the 7 series by pricing the upkeep of a 7 out of the reach of mere mortals like me. The car is very affordable to buy even as a BMW approved used such is the fearsome depreciation in the first two years....