"The problem is it all draws from the common reward pool; manipulating votes means you are pulling rewards away from other posts that are being voted on merit. If each sub had its own pool that would be different."
I don't see it as manipulating a vote and the post obviously has merit if it's getting traffic from the site and outside sources plus votes.
Do you have a source for the terms that says giving away items in a raffle style would be manipulative or is it mostly your opinion?
As discussed in PM, the issue of reward abuse is decided by community consensus, not any sort of explicit rules (who would enforce them on a blockchain anyway?). The major stakeholders decide what is abusive and voluntarily flag/downvote to discourage it. We get no rewards for downvoting and our only incentive is to maximize the success of the system (and therefore the value of our investment). This is in fact an explicit part of the design that is discussed in the whitepaper.