BH and Adorama are actually under $3000 too -- but they can't advertise it. You have to click "Add to Cart" to see the actual price they charge you (which sometimes is significantly less than the price shown next to the product).
One possible reason for the price difference could be that Amazon is starting to pay sales tax in some states. That would ultimately make their cameras (and other products) less competitive price wise. When I got mine , the sales tax made the camera (5d mark iii) just about the same price as BH Photo and Adorama. It is not a grey market version. I was able to register it for warranty support on the Canon USA site with no issues.
I asked the same question on another site - and they said there is a plastic model made and shipped from overseas - that's obviously the cheap one. But they don't make it easy to find out which is which!!