Here is a reason one might ethically believe Greenwald should be thwarted: because they believe that Greenwald is deeply dishonest, making up details about stories in order to fill in the gaps of his narrative and reporting them as if they were facts, so that half the Internet now believes that Google, the one big Internet company known to have actually invested real resources in opposing dragnet surveillance, is instead in league with the NSA, and instead of using Google Mail we should all support incompetent kooks like Ladar Levison.
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.