Speaking as an employer, responding to such a request opens one up to all sorts of legal challenges.
Net result: on the advice of our legal consultants we never provide feedback.
I personally think this is a tragedy, but it's a direct result of the litigious nature of today's society, and the aggressive nature of some people.
But consider this. If it's a small company and they responded so swiftly to code that you (and others) thought was fine, would you be happy working there anyway?
Of course, if you don't ask, you don't get. Why not phrase it carefully to show that you accept that you haven't got the job, but you are keen to learn from the experience, and would welcome any comments about your code, understanding that this cannot form the basis of any kind of action against them.