As @HomuraDidNothinWrong and @ToastyMozart point out, the REAL thing of it here, is that because it happened and was confirmed, that opens reddit up to a whole lot of potential legal pain.
In the reddit threads about this, for example, there was apparently at least one case of somebody in the EU being arreseted for comments posted to reddit. This now opens up an angle of defense of 'See? The CEO edits posts! I didn't type that!' which goes from "Here's the post you typed, as evidence" to a much more long-drawn out process of vetting data integrity of the post to editing and the like. It'd be made even more complicated if there were any stupid 'ghost edit' modes that allow mods/admins to edit posts without logging the change. One would hope nothing like that is implemented, but reddit's always had some curious systems choices.
The other thing is, once you start editing comments outside of rule-based moderation, it, as Homura said, opens you up to losing fair harbor protections, meaning anything posted could be considered 'vetted' by staff and therefor their speech. Which, if you recall the sorta stuff gets posted on reddit all the time, could open them up to an obscene swath of lawsuits, from leaked nudes, libel and so on.
Are the original posters a bunch of dickbags? Sure. But a non-idiot would have just deleted the posts and/or shadowbanned the offenders and moved on with their day. Instead, the CEO pulled a profoundly, obscenely stupid move and has opened the possibility of a legal buttblasting.
Which probably wasn't worth the 10 seconds of 'haha gotcha' the edits yielded.