If you asked for an hour of my time, you'll get an hour of my time. If you want an extra 15 minutes to chit chat after, that's fine, just ask for it ahead of time so I can plan for it.
Whatever expectations you hold a candidate to should be reciprocal. So if you're going to hold candidates responsible for blocking off extra time before and after their interview to sit and chat, then you should be doing the same. Which means you should decline any meetings that are scheduled to start within 15 minutes of your interview ending. If you do that, then great, that's wonderful for the candidates who have more time to spend. You should also then tell the candidate up front to allow for 15 minutes on either end of the interview.
This whole idea that a candidate must have a buffer around their interview with you or they don't want to work for you is nonsense. It makes no more sense than saying if I have a 10am, 10:30am, and 11am meeting then that means I don't want to meet with the person at 10:30am. People have other things going on in their life. Employers who realize this, and respect candidates during the interview process, are going to have an easier time hiring good candidates that will also treat them well in return.