There's this from @steemitblog. The idea is to get rewards to posts the community likes rather than where people buy votes. A lot of vote sellers have switched to manual curation and they can still earn as they get more curation rewards. If you just want to promote your post then decline rewards and it works like other forms of promotion such as buying Google ads. It is not a rule as such, just the way a lot of the community wants it to go. Of course some will continue to milk Steem for all they can get, but we have our free downvotes to use on them now.

I only downvote to remove unearned rewards or in rare cases where someone gets abusive.