Because, if you want to tell your potential bidders in your listing that part of the winnings will go to charity, the rules at eBay force you to choose a listed charity. You cannot simply state you are donating some of the funds to… oh, say… the Katrina Fund way back when Katrina struck.
I learned the hard way, stating in many listings that I planned to donate ten % of all winnings. I got reported by competitors and the listings removed. Since all the multiple offenses were a first offense I got off easy. But all the listings were pulled and I had to re-list after taking out all references to charity.
Apparently some good fellow at the bay management thought it was just too tempting to sway bidders to your listing this way and then maybe not even give any to the Katrina Fund. (It was like they were reading my mind).