His arguments and reasoning were edited for tone and posted by others and HR did not take any of that down. He wasn't fired for what he said, but how he said it. His post was vicious and demeaning toward other FB employees. It directly attacked the character and ethical constitution of specific employees in the company.
That said, HR did later revoke open access to some of the specific details he used to make his case (conversations around investigating and escalating user reports). FB gives employees access to much more than most companies, so the sort of things HR made secret would not be visible at most US companies.
Although I think at FB there are many things you can't say, his actual beliefs were not bold or rare. Many people say similar things all the time. But you can't go around attacking people like that.