What they mean by unsustainable is that for the price it costs them to stock these means selling them to consumers at the existing price would not make any money, and the amount of money they’d have to raise the prices by in order for it to be profitable would stop consumers from buying it altogether.

Essentially, there’s no way to sell them to consumers in a way that will make money. Therefore they have to sell to big corporate customers in order to make any money at all. These companies are not lacking in corporate greed, but in this case there’s literally no other option.