If someone clearly isn't right for the job, you owe it to them to mention why- I'm an engineer, I work in absolutes and "Culture fit" or something so wishy-washy doesn't roll well off my fingers.
But how do you tell people that they lack basic knowledge or experience in certain areas without sounding bitchy or attacking.
Even worse is the ones that interviewed well, or showed significant competence.. yet were not as strong as another candidate.
I agree it's wrong to send nothing- but it can be hard to formulate a pleasant email.. especially if you've interviewed many people. (10-15 at my last employment, I was replacing myself and had limited time to document/check everything).