Very early on at Netflix, Reed Hastings wrote a culture memo for the company with Patty McCord, then our head of talent. At its heart was the notion of integrity and feedback, which they described as "You only say things about fellow employees you say to their face".
What happened here was unfortunately not simply venting on Slack or a single conversation. These were critical, personal comments made over several months about their peers (not their management as suggested by The Hollywood Reporter) - including during meetings when those peers were talking or presenting. This is entirely inconsistent with those values, which is why their manager fired them.
"Jon," you might be thinking, "this sounds an awful lot like protected concerted activity under Section 7 of the National Labor Relations Act. You've spilled a lot of digital ink over the years telling us why we can't fire employees for what they post to social media for that reason. What gives?"
You are correct that Section 7 of the National Labor Relations Act grants employees the right to engage in protected concerted activity — the right to talk between and among themselves about wages, hours, and other terms and conditions of employment. These protections most definitely can extend to your employees' gripes about their bosses in person and online. Without seeing these employees' Slack posts, I have no idea whether they meet this standard or not. They could, but I just don't know.
What I do know, however, is that these three fired employees were not "employees" covered by the National Labor Relations Act; they were almost certainly "supervisors," which the NLRA does not cover.
According to the NLRA, "The term 'supervisor' means any individual having authority, in the interest of the employer, to hire, transfer, suspend, lay off, recall, promote, discharge, assign, reward, or discipline other employees, or responsibly to direct them, or to adjust their grievances, or effectively to recommend such action, if in connection with the foregoing the exercise of such authority is not of a merely routine or clerical nature, but requires the use of independent judgment," and the term "Employee" does not include "any individual employed as a supervisor."