Why does eBay remove fair and appropriate negative feedback for unreliable sellers?

[foliogaga]

I have had two experiences where eBay business sellers (with large turnovers) have 100% positive feedback and yet who provided me with very poor service with faulty or damaged product, who then refuse to acknowledge their error and refuse to refund return postage costs - even when photographic evidence of the flaws are provided. I then find, to my amazement, that when I leave entirely fair and appropriate negative feedback, describing accurately what occurred, this feedback is removed by eBay, presumably on appeal from the sellers. How can we have any faith in sellers, even with 100% feedback, if eBay sides with poor sellers rather than the buyers who have been badly treated?