Because they can negatively impact your feedback score, and the difference between a 99.8% positive and 100% positive feedback score when you're selling items is a pretty big deal. A lot of people (rightly or not) won't buy from somebody who has less than 100% feedback.
I understand the hypcracy and how they hold you under the gun. Which is why I choose to elect to NOT give them not only my time but also my money. I refuse to have to nuzzle someone's crotch simply because they're too stupid to breathe.
So I guess in this case the seller is screwed because the buyer is an idiot.