Now, some important caveats:
(1) It is obviously possible to unethically thwart any writer's agenda. Publishing things you know to be untrue, even in the service of what you believe is a higher truth, is unethical; it's exactly what someone might accuse Greenwald of having done. For that matter, invading Greenwald's privacy by, I don't know, stealing his bank records would also be deeply unethical. But Palantir hasn't been accused of either of these, or, for that matter, anything else unethical w/r/t Greenwald. Feel free to enlighten me here.
(2) If you can't tell, I'm not a major Greenwald fan. Having said that, I also believe the best antidote for bad speech is good speech, so orchestrating a campaign to suppress him doesn't seem like a good strategy to me. It apparently sounded like a good idea to one of Palantir's SEs, which ended up getting the guy fired.