I can see where it might be problematic to go into reasons why they didn't get the job, but I'm having a hard time seeing what risks a simple notice informing them they didn't get it causes?
This may be enough to give the rejected applicant ammunition for a law suit. I've been to several interviews where I've had to dig out that what they say they're looking for doesn't match the position they actually want to fill.
I'm a straight, white male in my thirties, so discrimination is far from my mind. Other people may form different ideas, though..
On the other hand, going silent after there already has been some communication established does not seem to be smart. But I am not a lawyer and there could be some sense in that too.