It bums me out when sports look fixed. A Sports organization has a major credibility problem when bookies flip the odds upon the arrival of a referee, umpire or judge.
I'd offer it's because we are generally illogical, prone to believe in anecdote and circumstantial evidence (to put it generously), etc.
There have been very few true examples of fixing. But, one bad game, and NBA fans think the league is corrupt, refs are bought off, games are fixed, etc. etc. etc.

