If I gave this level of feedback for every candidate I talked to, I'd never get real work done. I think there's also the idea that we don't want to burn any bridges or upset you. It's easier and safer to say "You're not a fit" than to say "Your code isn't good enough, and here are all the problems we found with it." If you take that poorly, then it stresses everyone out, and I'm at risk of getting irate emails from whoever. Furthermore, I don't want you to be badmouthing our company because you think your code is the shit and we disagree.

I think the best way to get feedback on a rejected code sample is to give your code to an objective third party and ask their opinion, like OP did here.

For every 1 candidate who would take it as constructive criticism, there are 19 that would react hostilely or think it was the next step of an ongoing debate. Read some of notaio's responses in this thread and remember that he started this discussion with the express purpose of soliciting feedback.