Bidbots and vote sellers want to double-dip. They already get rewarded for curation, but they want you to pay them for a vote that isn't guaranteed to even return the value you gave away. It's most beneficial to the seller, and perhaps benefits the buyer, but it dilutes the reward pool from organic curation.

*edit* and there you were, saying what I was typing right after I hit the reply button. Shows me what I get for not listening all the way through first.