Pretty shitty to do this to anyone. The only scenario where firing someone for (legally) using a certain software should be allowed to happen is where it was communicated clearly in advance that using that software is not allowed in that particular job position.
IMO the strongest plausible interpretation of what the comment you’re replying to said would include the Linux Foundation clearly communicating the new practice beforehand.
Why? Writing a tool - even an operating system kernel - doesn't mean that that tool is appropriate for everything (for instance, for everyday use). Linux does many things great - a fair number of them industry-leading - but desktop user experience is not one of them and has never been close to being so.
Sure, but I still think that the Linux Foundation should represent Linux and not Microsoft.