Well, I guess the "bounty" is more about giving you extra priority by providing it on the front page. So you must "pay" for that, it's not about the answer itself. It's like advertisement on a web page: even if nobody clicks on the ad, people who wanted to advertise something had to "buy" the place that ad is shown. I am not related to the askubuntu.com team or such, but I guess this is the answer: bounty "fee" is about handling your question with priority (like showing it on the front page) and it's nothing about the answers. This is my idea on the topic, let me to know if I am wrong.