There is a very clever piece of persuasion at work here. Not only does it soften the approach to the customer from a sales approach to a customer service approach, but it also make it easier for the flight attendant, who may not be so comfortable with the notion of selling. This is an important consideration when attempting to add a cross-selling function to staff who signed up because they liked helping people but never wanted to be in a sales role.
I asked the flight attendant if she was trained to ask in the specific way, and she said she wasn't. Well, that was a disappointment! Was really hoping that there was some evil persuasion professional deep in the bowels of the airline marketing department who was conspiring to break down my aversion to paying for food on a flight. It could still be that she preferred the less overt phrasing, or that the marketing people chose the label "food-for-purchase" for the reasons I've said.
Like it or not, we continue to be bombarded with more and more marketing messages everywhere we turn, and when we least expect (or notice) them. Whether you're a marketer or a consumer (or both), it's good to have a greater awareness of them and how they work.