Nothing incentivizes it specifically, this change is more about removing the disincentive from downvoting (which right now is that if you downvote, you're losing out on potential curation rewards).
I'm not sure how an incentive model specifically could work for down votes, but at least removing the penalty for down voting would (IMO) make for a better system.
But that's my point, we ARE inherently disincentivized from down-voting, regardless of whether you remove the voting power penalty or not - it's the fear of retaliation (flag wars) which indirectly hits you with a voting power penalty in the form of having rewards downvoted away by those that we downvoted in the past.
So my suggestion is that we counter that inherent disincentivization (or the natural tendency for the majority of people to side step the possibility of voting retaliation, or "flag wars", by simply not participating) with some form of incentivization, something like an SMT that rewards "smart (useful to the system) downvoting", in an attempt to convince a larger portion of the population to play this game in a useful way, which actually might bring something like a "real consensus" as to what is good or bad content.