When I've fostered dogs and interviewed potential owners, I've always asked what they planned on using for food. I always adopt dogs out with at least a 6lb bag of food, and explain to them why they should continue to keep the dog on a high quality food.
If they grumble and say "well, I was planning on feeding Beneful/Pedigree/wood pellets/etc," I try to educate them on why it's healthier to feed a better food. If they don't listen or care, well, no dog for you! I don't care if you're feeding the most expensive food on the market... BUT if I'm adopting a dog out to you, you'd better be feeding something nutritious, whether it's Kirkland or Diamond Naturals or Taste of the Wild or raw, etc.
I think it's silly when a breeder says "you need to feed THIS food. Oh yeah, and I'm the only supplier in the state and you must order from me otherwise your health contract is void. And who cares if the first ingredient is corn. You MUST feed this," etc.
However, I think it's completely reasonable for a breeder to expect their puppy buyer to feed a high quality diet. I would never sell a puppy to someone who planned on feeding Ol' Roy.