I have never received any meaningful feedback from prospective employers about why they declined to advance me through their interviewing process, so I feel no

I have never received any meaningful feedback from prospective employers about why they declined to advance me through their interviewing process, so I feel no reciprocal obligation to give any feedback about why I didn't advance them through my interviewing process. In fact, if you base it strictly on reciprocity, for some companies, you wouldn't even have to send an e-mail to the recruiter. You could just never contact them again and leave them to wonder. But I consider that behavior to be so beyond the pale in prospective employers that a short, e-mailed note saying "I decline your offer," is the minimum that I would ever do.