Why is food as a reward so reviled by dog owners?

It never ceases to amaze me. At a new off leash park today, Giro picks up a ball a Golden Retriever didn't bother picking up. Wanting to return it to the owner, I ask Giro to drop the ball off with me, which he promptly does. I slip him a treat to make up for the fact I won't be throwing this ball again despite the perfect delivery. "Oh, your dog does that much better than mine!" observes the owner. I don't think she saw the treat I gave him. Then, as her Golden hopefully nuzzles my hand, she spies the treat. "Mine is absolutely terrible with food! Can't stop him!" "Aren't they all?" I reply, but clearly, in her mind her dog is a hopeless case of a food addict. He'll never learn to be a proper dog, who only cares about pleasing his owner, and brings the ball back properly without being trained to do so. And yet, this dog, and so many like him, would be a snap to train by positive reinforcement. In the process, he would learn to really adore his owner, because he will experience so many good things through them. So although the dog will initially work for the food (and will continue to do so), he will also start to enjoy the game of training. Later, being asked to do some tricks he knows and loves becomes a treat in its own right. Maybe if people had a better understanding of this aspect of using food in training, they might look upon it more favourably.