"You were sufficiently promising to pass our screening but after meeting you we can definitely do better. Have a nice day." is not an easy thing to say even in absence of legal issues. I could tap-dance on your face as an employer without getting sued here, but that doesn't mean we sent applicants home with a list of their inadequacies.

Of course, if you believe social science research, the reason is probably "You failed in the first five seconds to connect with your interviewer, he rejected you then, and he eventually found a reason to hang his hat on."