I've been thinking it over, and I think you're right. If this sort of public response were the norm, the nature of the internet means that any CEO of any company would hold quite a lot of power over the entire future of every employee.
Right now you can do poorly at a programming job without it affecting your entire future. That's important because people have down periods in their lives. It happens.
If you get fired and go out to a bar and tell people that you got screwed by the scummy company, that's pretty normal. And why not let someone feel better about themselves? There's no reason for anyone else to believe their stories. Feeling like you got screwed is better than feeling like a failure. Depression can ruin your life if you don't find ways to cope, even if they're irrational.
Let's say he even convinces someone not to go work for Reddit. That sucks, but the potential employee should've realized that of course the former employee who publicly admitted he was fired is going to say something bad about the company. It's just one side of the story.
I was recently shocked to accept that only 17k people voted on the Obama AMA, even though 3M people viewed it. The disparity between participants vs lurkers on a site like Reddit is so large that it's hard to fathom. How many people saw Yishan's response? There are 4k upvotes. Applying the Obama AMA ratio suggests 700k people read it. Realistically it's probably closer to 40-100k, but still, what if it was common for everyone to go on the internet and badmouth employers and employees and so on and 0.1M people get to watch? A website like glassdoor, but for companies to go and rate their employees for all the other employers.
Yishan's reply to David generated over 1250 responses. David's comment saying "I was laid off" in total generated just shy of 5k comments. How likely is it that out of 40k people who viewed this, 5k of them left comments? It seems entirely possible about half a million people watched the CEO of Reddit lay the smackdown on a former employee who didn't even really say anything too out of line. Distasteful and in bad faith? Sure, but it's not like he was saying Reddit is an awful place to work. Envisioning a future where this is commonplace and people jeer from the sidelines like http://i.imgur.com/kQxF02o.png isn't a reality I'd like to live in.