What's wrong with that? If they leave feedback people can argue and claim it's invalid. Interviewing at a company isn't a free mentoring resource.

Professional or even human decency?

Candidate just devoted significant portion of an entire day with no compensation in hopes to work for you, you basically say 'no' and give nothing back.. its using peoples labor for free and trashing them.

Interviewing candidates also isn't a free labor source (in event that 'homework' has some professional use, which it often could)

Because it's classless. You inveigle a candidate to spend their time on a test you owe them the courtesy of feedback.

I once spent two hours on a test for a contract development company. I'm an accomplished senior developer and normally i would have told them to F off, but I was curious so i did it. I did pretty well on it, but never got a response. Zip, nada, nothing. I emailed them multiple times and even called HR. Still no response.

Anytime other devs ask me about that company, i say don't waste your time, they are super shitty.

Everything.

Other than obviously or blatantly incompetent coding, I'd expect that as a minimum, a code review would be a part of the equation.

This is similar to you working on a project, your boss looking over it, not saying a word, dragging it to the Recycle Bin, and walking off.

See it as minimum payment for the time the applicant spent on it. (feel free to disregard if you pay them for the time they spend on the homework with actual money)

I get your point, but this still applies to the binary tree... That's the interview process. Yes, companies can make a better effort to give feedback, but I think that's secondary to give me the right opportunity to show what I can do.