Do we need a direct incentive to flag? No. Just like a restaurant owner has no direct incentive to keep their store clean. They get paid to sell items on their menu. There's a direct incentive for that. There is no item on the menu for "clean the bathroom."

https://steemit.com/curation/@inertia/now-you-see-that-steem-power-will-always-triumph-because-influence-caps-are-dumb