It's a proxy for what the stores have a hard time accomplishing. The goal is to fulfill demand. Do you genuinely need the explanation why a store's goal is to sell not "have on shelf"?
> From the store's perspective it is still in stock and supply is available
I guess I do need to explain. Now I'm starting to think this is intentional hard headedness :). If the goal was just "have X on the shelf" they'd put one item and price it like gold. Or put it really high on the shelf. Goal achieved. When the authors of these articles say "have on shelf" they assume the reader is reasonable and intuitively makes the connection to "fulfilling demand, people able to buy".
> How much people eat of it is separate from what the article is discussing.
You're right. What the article is discussing is how much people can buy. If there is no supply then people aren't buying. But make no mistake, the buying is important, not the being on the shelf. Maybe some stores put the breads in a basket ;).