It is amazing how strong the feelings are on both sides of this...I'm not much of a cook, so it never really occurred to me.
However, if one removes the word "recipe" and substitutes "play" or "novel" or "quilt" or any other creative endeavor, it becomes clear that, to many, their recipes are more than just formulas for a successful meal- the recipes represent hours of effort as well as substantial monetary investment. Just as any other creative endeavor, the person who created the work should be able to control how it is used, and there is nothing "selfish" about that. (Edited to add: I'm not necessarily talking about copyright law, just the idea that the creator of a work has a continuing interest in the integrity of that work, legalities aside.)
Honestly, I would be flattered if someone asked me for a recipe, but recipes aren't that important to me. And they don't have to be! However, if someone came up to me and said, "Hey, you remember that play you wrote 10 years ago? We're performing it next week!" you bet I'd be upset...not that it is being performed, but that it is being performed without my permission or input.
I don't think you should be offended if someone asks you for a recipe, but the fact that someone asked doesn't mean you are obligated to give. If someone says, "No, I don't share recipes," that is sufficient and shouldn't be argued with. If you think that stance is silly, that is your opinion, but it is in no way backed up by eitiquette.
I have yet to see any etiquette expert advise that one must aquiesce to any request, for whatever reason, if one is uncomfortable doing so.