Just my opinion, but I suspect their profit margin is the motivation. The parts for the new rudder probably cost them $150-$200, and they're providing them to you for free. That's probably most of the profit on an empennage kit. If they wait until you buy the wings, then they take a much smaller loss percentage wise, they can save on shipping by including it with your wing kit, and they can be reasonably certain that you're going to buy fuselage and finish kits in the future.
PJ