According to the Zappos' CEO blog, here's their reasoning for
completely removing price protection.
"We decided to remove our price protection policy from Zappos because we didn't feel it was consistent with our goal of making Zappos customer service-focused as opposed to price-focused. We found that most of the customers that were using our former price protection policy would actually save even more money if we simply started out with more deeply discounted products in the first place. Instead of on Zappos, you'll be able to find those deeply discounted products on 6pm.com."