I've only tried this a few times and I seem to get silence in return.
This is only after the phone interview; after I get a "Thanks but no thanks" email I've asked in polite terms why I got passed over. I've heard back only once out of the few times I've done it and I felt that it really was a kind of hard decision for them to make.
Is it impolite? Is it a legal concern? Do they not like me that much? What?
I'm guessing the lack of a response is because:
1. They don't feel they have to justify their decision. 2. They don't want to get into a dispute/nit-pick. 3. They are just bad hirers and don't know how to provide adequate feedback. 4. They are trying to hire someone, not help people they are not hiring.
Conversely, when I have turned down an offer those companies have never asked me why.
- James.