how are they evil? I feel like I've missed something.
Anything awful the NSA do for patriotism (however misguided), Palantir do for money.
The name is a dead give-away. In Tolkien's stories, the palantir is a crystal ball which corrupts the mind of its user. At the least, the name strongly suggests that they lack a proper respect for the dangers of the technologies they're using.
That doesn't really explain anything.
I had a read of the article that was linked to from that Quora answer and I still couldn't glean 'evil' from that -- potential to become evil, yes; actively intending to do evil, not so much.