If you want to compare, then imagine that you're in the middle of the interview and the interviewer suddenly tells you “get lost, here's the elevator”.
Saying “thanks but no, thanks” after the interview without telling the reason if perfectly fine for both a company and an interviewee.
The candidate didn't do the equivalent of "get lost."
I have been flushed from an interview process halfway through. (Apparently I failed the personality test for Capital One. Fair enough.) They had a car take me back to the airport. I didn't mind. The guy I was riding with (who was also flushed) was really pissed about it, though.
A company really should solicit feedback from the candidate during the process. The candidate can say "I don't think this company is going to be a good fit," and then the company can decide if there's anything more worth talking about. Maybe there is, maybe there isn't.
If you want to compare, then imagine that you're in the middle of the interview and the interviewer suddenly tells you “get lost, here's the elevator”.
This happens. The reverse isn't unreasonable either.
A lot of companies schedule interviews with a natural break in the process partway through -- basically you do phone screens, with people not progressing beyond those, and the in-person interview can be split to morning, lunch, after-lunch; senior people (or larger groups, or people with more constrained schedules, ...) meet with candidates after lunch, and if someone is a no-hire based on the morning, the candidate doesn't know about the afternoon phase and is told he can go home before or after lunch.
Personally I think it's important to sell no-hire candidates on the company, since even no-hires might be hires in the future (for different roles, or if they add skills), or might be referrers of other, better candidates, or might end up working for a vendor or customer, or just might make negative social media postings about you which dissuade other candidates. So doing morning interviews, lunch (which turns into "selling the company's mission to the candidate", ideally by a senior person who isn't overly busy, not a random HR drone), etc. for the no-hires.
Saying “thanks but no, thanks” after the interview without telling the reason if perfectly fine for both a company and an interviewee.