Remember that the actual profit a dealer makes on a new car sale is often a relatively small difference between two very large numbers, and whilst a full tank of fuel might seem like small change compared to what you've just shelled out on the car, it could represent a significant percentage of the dealer's margin.
Also, just how much fuel did you leave in the car you left behind as a trade-in?
That said, I do think a new car should be handed over with at least enough fuel to enable you to get home. I got over £2k discount off both my cars, and was still given at least a quarter of a tankful of petrol.