Former Reddit CEO Ellen Pao made a good comment on the controversy: https://reddit.com/r/announcements/comments/5frg1n/tifu_by_e...
I think this guy needs to grow up.
Basically, by not firing him, the board is sending a message. It is saying that 1. The platform and it's comments cannot be trusted. and 2. That it's ok for employees to surreptitiously edit comments. If the CEO did it and didn't get fired, then how could they fire a lower level employee for doing it?
The CEO has set the standard through his own behaviour. And that is now the culture of reddit. By not dismissing the CEO, the board has sent the message that they accept this behaviour.