Seriously, I can't even believe that. As a chef I ALWAYS have helped other people in the kitchen and it enrages me that other people who enjoy cooking or do it on a professional level don't do this in a larger sense of the philosophy. If you had threatened to sue them or demanded some sort of payment I might have understood the response you received. However suggesting that to learn their lesson they should apologize (totally deserved and there was a spot recently on NPR about how sometimes you gotta nudge someone to apologize lol) and donate money (to a damn good cause I might add, thank you for being unselfish), that was class IMHO.
Let me just tell you a little story related to people and their understanding of the Internet;
Yesterday I was chatting with someone online. We got into a discussion about piracy of software (I am completely against it and the other person seemed to claim that he/she wouldn't do it but that it wasn't wrong?). The statement made was; It is OK to download something on the Internet early as long as you are going to purchase what you download when it actually is released in stores. A friend of mine brought up that would be like stealing a car and then going back and paying for it a week later. The person's response to this; you can't compare killing a family of 4 to killing a mouse.
Now obviously the conversation that took place was much longer than that but I will save you the unimportant details lol. But this is the stupidity I think a lot of us are forced to deal with on a daily basis.
Unfortunately people who are less comfortable with technology want to do whatever it is in their power to smite those of us who know how to use it and have an easier time at getting x, y or z accomplished thanks to technology (i.e. I would personally see your Internet postings as published even though they are in digital format, and I am sure it was easier than having to send a million type-written copies to 1000 different editors to attempt to get the same result).
Anyways now I seem to have lost my train of thought, arg! Needless to say I would be happy to support this cause if there were some sort of donations needed to cover your legal fees in this matter. Maybe set one up so that you can give this person what they have coming. As you said, this is plagiarism, no if's, and's or but's about it. Time for this person to learn a valuable lesson...Just because it is on the Internet does not mean you can take it and do whatever you want with it (creator be damned).
I really hope you can get this resolved and the person ends up learning their lesson.