How frequently do you replenish your spices/WD40? If you are a restuarant, then a button would make sense. The dash, in its current form, seems suited to high-turnover products where the sales generated over the devices lifetime can cover the initial cost.
As someone else mentioned though, think of it as "Dash, the physical button" + "Dash, the API". The API is probably the long term goal, the physical button is just a (temporary) drop-dead simple API client that people can use today. Amazon is likely betting on smart-fridges/-pantries/-homes catching up in the future
Edit: in the near-future, I imagine Amazon adding a touchscreen (so it's not product specific) without adding much to BoM. They can also charge manufactures for on-device screen-time (who doesn't like ads that convert to an immediate sale?)