I would never, ever say "Joe, can you give me the cell phone and home phone numbers for the chairman, chief pilot and CEO of the Trans Global Air Lines?" Even though I know he had those numbers in his wallet. Obtaining them was the fruit of many years of source cultivation and rel@tionship building, and they were not to be shared or used lightly. We all have similar information in our respective contact files and even among friends and co-workers of long standing, it's just understood that you don't share it and you don't put someone on the spot by even making the request in the first place. In fact, asking for the information is such a faux pas in the first place that pretending not to hear it, as the OP's friend is doing with the cupcake recipe, is probably the most kind and polite response!
This makes absolutely perfect sense in the context of your analogy, because it truly is privileged information, and nearly everyone would (or should) see that without needing it explained. It's a pretty commonly accepted fact that unpublished numbers are private information, and should not be shared without the owners' explicit permission.
But even though I agree completely that there is a difference between a recipe off the back of a currently available food product and a person's signature recipe developed after some amount of effort (even if it was inspired by a readily available published recipe), I do think there is a bit of a difference between this situation and your analogy with the phone numbers.
The difference is in the fact that there are just as many people in this world who view recipes as "share and share alike" as there are people who view recipes as proprietary. Neither view is so universal as to make the other unusual or wrong. It would be just as silly for me to assume that someone wouldn't
want to share a recipe as it would be for me to assume that someone would
, unless I had information that indicated one response was more plausible than the other.
From what CoffeeKat has said about the situation, I don't think it's fair to say she had more information that indicated her friend wouldn't share the recipe than she had indicating that her friend would. She had one piece of information indicating her friend might want to keep it to herself: the fact that she beandipped a casual acquaintance. But she had a lot more information indicating her friend might be willing to share: the closeness of their friendship, their distance from each other, the fact that her friend had asked CoffeeKat to share recipes, and the fact that CoffeeKat had never actually asked for the recipe before.
Given that, I really can't see how the question itself was rude, especially since CoffeeKat did make it clear that she was perfectly willing to accept "no" as an answer.