The point is to make it clear and certain that people should not vote up Trending posts on the basis of curation rewards. You are correct that the rewards are small in practice and that likely the smart thing to do anyway, but the formula is complicated and even if people understand it, they may still throw up a vote thinking that a chance to get 0.001 SP is better than nothing. When it is unambiguously zero then it is removed altogether as a reason to vote up those posts.