Unfortunately it's not basic; the ability to strip all context to make this about being mean to a customer makes for a poor argument.
The government isn't just _any_ customer, whether you think the firing was justified or not. The DoD enjoys a ton of taxpayer budget money and a sanctioned right to kill people if it deems it necessary. And it was proactively recruiting at a tech conference for software that isn't innately violent. Is that bad? Maybe not, maybe so. Is that a basic customer? Not at all.
Is this really only about criticizing the government at work? If she made the tweet off hours or wasn't even at the conference, does Nicholas Chaillan still complain to VMWare and get her fired? When is it okay for her to criticize the government without losing her job? Would it even have cost VMWare anything if they didn't fire her and just disciplined her? How about the social media policy that bans talking about incredibly generic things like "age, sex, race, religion"? Has anyone at VMWare ever gotten disciplined for tweeting out "Jesus lives"?
I think your reduction to "employees must never tell any customer ever to go fuck themselves and that's basically what happened here" isn't useful nor is it one of the more interesting questions this post raises.