Yeah, I saw that letter this morning, and it sounds like something one particular friend of mine would do (tried to do).
Seriously, the host wanting to mix the store-bought and homemade just shows that she really had no concept or appreciation for the effort of the letter writer. They are totally taking advantage of the letter writer, and even if they are told straight out, I doubt the "hosts" would agree that they did anything wrong.
I no longer willingly participate in any "potlucks" or parties with the friend who tried this on me. We meet at restaurants for dinners now. Even that gets to be too much sometimes, but it's much more pleasant than being told what to make, or once I tell what I plan to bring, having this friend make something super similar, then trying to get people to say how much better hers is...