If you buy a used game for $40, you paid $40 to the publisher while the person who sold you the game only paid $20. That person will likely buy another game from said publisher when the next game comes out because he had so much fun and only spent $20. A few months later you sell for $20. Last guy decides it's not worth to sell again. What happens here is EVERYONE paid $20 for the game. Everyone is equally contributing to the publisher. If nobody buys used then way less people will buy new, knowing they can't sell it. So buying used help publishers sell more copies.
Why doesn't the game drop to $20 immediately? Because there aren't enough copies to go around. Buying used take a copy out of circulation and drive the price up. So again, a person sees that used copies are $40, they are more likely to buy a game new.