Notice the word is "accused" and not "guilty". Now I am not going to accuse YC of running a star chamber: I assume (without knowing) that they are fairly responsible with their blacklist. But the principle is still worrying.
Like most institutions, private companies are not able to hold trials and rigorously tell guilt from innocence, so whenever they set out to police others they must do so on some lesser evidence.
It is our right to shun others because we suspect them without proof of something bad. Perhaps it is even our right to gang together and ostracise such, but society suffers when that is the norm.