You have to see it in a historic context. Sennheiser was once a manufacturer of high-quality consumer-products sold by many dealers. About 20 years ago, those dealers died and were replaced by a few hugh electronic-stores (in Germany it's the Metro with it's Saturn and Media-Markt brand, in the States it's propably Best Buy or Wal-Mart). They don't have skilled sales-personal which are able to communicate the differences between Sennheiser and cheaper brands and they have ridiciculous contracts. They order thousands of headphones, send most of them back after a few months (which weren't sold), pay about 10% less every year...
That's when Sennheiser decided to cut costs and quality, outsourcing production to Ireland and now China offering headphones from 3-300$ with high margins.
The HD800 is the first "professional" product in the headphone line-up in years and uses the very same production technology/standards as their professional microphones (which aren't sold in electronic-stores). But to protect themselves from a dangerous cost-cutting they had to make tight contracts with the dealers, otherwise you could propably order HD800 from Amazon for 10-20% discount already. That's lowering the value of the investment of previous HD800-buyers and would make the HD800 just another piece of electronic-junk.
Rebates are drugs: first you enjoy them, then you become addictive (you don't accept any retail price anymore) and then they destroy you.
It's impossible to offer high-quality products this way.
They're made of plastic which has a much lower E-modulus ("stiffness") than regular metal. But as a mechanical engineer I can assure you that despite design/silver-look they're definitely on the upper end of mechanical quality - very tight tolerances and assembly. The plastic parts are most likely manufactured by top-notch suppliers from the German automotive industry with high-end molds and tightly controlled injection processes - something that you won't find with Grado or Chinese OEMs...