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