But what possible (noneconomic) reason could they give for barring negative feedback on buyers?
I think the reasoning is: if the buyer doesn't pay, the seller hasn't lost anything [other than your time/effort on the listing, of which ebay cares not]. You still have the item to relist, so you just move on. I can see both sides of this fence...and it is frustrating to not be able to "out" the buyer, I agree.
Also, though, ebay seems to have thrown that whole "legal binding contract" thing out the window when they changed this policy, which I think was quite some time ago.