As far as the editing Spez did, if they were really out there calling him a pedophile, they should have expected some backlash. You can only push an authority figure so far before they're going to go on the defensive. This will definitely make things worse though.
I didn't know that was so common on SA. I knew the site was heavily-moderated, despite The Goons' famous repuation. But it's still terrible there, too.
but he didn't mess with any of the content of any messages.
If he did, the entire chan would have gone up in flames.
Banning Pizzagate (as absurd as it is) would definitely have created an uproar, though, much like banning Gamergate did.
Reddit is exactly the same. Keep pulling shit like this, and it will drive quality users away out of principle. But Zombie Reddit will keep lurching forward, spewing poorly compressed Facebook screencaps and cat gifs.
But I do maintain that that's a little different. That's essentially another instance of "disregard that, I suck cock," whereas spez actually altered people's stated opinions.
Some subreddits may be "wretched hive of scum an villany", you can't say that the entire website is like that.
Reddit (the company) is a wretched hive of scum an villany, where this sort of stuff seems almost regular. Perhaps it wasn't always so: I don't know.
But I can't think of any other forum that would put up with this. If an owner or mod did this on the *chans, the chan in question would be abandoned within the week.
As for HN, dang does as he sees fit, as do the admins: they'll split your thread, redact your post (and if they doesn't, the software will if it gets flagged enough, IIRC), and (or so I heard) even shadowban you of you're bad enough. However, they're mostly right, and not corrupt. And they wouldn't pull crap like this. Blocking a comment that talks about how HN is just a massive circlejerk is one thing: actually altering it to say something else is quite another.
I suppose it might be common on forums run by egotistical gits, but come on, you're heading a company running a large forum/news aggregator. You can do better than an egomaniac with a website.