While I appreciate the OPs comment regarding first class passengers being able to buy Y food, I feel the FA was probably looking out for the Y passengers. There are only so many items catered for Y, and it would be unfair for passengers in first class to get them in preference to Y pax, especially when first class pax already have meals catered (that can't be sold in the back). All that being said, the FA should have offered to bring a sandwich after all Y pax have had the opportunity to purchase one.