So, you'd regard a candidate as lacking integrity if you scheduled an interview with them, but they then cancelled because they'd accepted another offer first?

Yes I would and this happened when I was on the other side. The candidate reached out after 2 weeks and asked if the job was still open and that he had made a mistake. The job was filled.

So, knowing that he was going to take another job, you'd rather he still came to your interview and used up hours of your employees' time? To me, that's the action that would lack integrity.

Your example of the guy who made a mistake is certainly a bummer for him, but presumably he didn't know he was making a mistake when he turned you down. And that's got to be a very small fraction of hiring situations.

I think the integrity test came when he made up his mind and accepted an offer before the interview. If I knew he had already accepted the other position then it would be a waste of time.

The problem wasn't that he cancelled the interview, it was that he made up his mind before a scheduled interview with me.