The point is simple, someone may want to make a post, but not make money from it.

This was a popular feature request, so it was added sometime towards the end of last year.

You'll have to ask authors who decline payouts why they do it. For example, developers may have announcements and since they are well paid already, they may choose to take an ethical high ground. By doing so, they donate their rewards back to the reward pool so it's redistributed to other authors.

In the case of the post that you screengrabbed by @krnel, you'll have to ask them why they declined. You can leave a comment there, he may reply.