"I have long wondered why witnesses don't decline payment on most of their posts?"
Not only is it very rare to ever find a witness who will actually decline payment for their posts, but between them and Steemit's corrupt management, they have even tweaked the curator payout calculations so as to pay themselves 99% of the reward pool, and leave essentially nothing for the rest of us. Also, in order to boost their own earnings even further, they setup multiple accounts to downvote numerous other members to the point that their reputations are completely ruined and they receive nothing from the curation reward pools. And one of the worst of such abusers is @themarkymark despite the fact that he is currently in the #3 spot on of the Steemit witness chart. It's rather unfortunate that most Steemit members are simply too naive to know when they are being screwed, and ironically vote for the very same people who are screwing them out of their rewards.