What you are saying would make a lot of sense to me if manipulating people into buying things had nothing to do with their emotions. While we are accusing each other of being callous towards mental illness, I'll note that anyone who has developed an eating disorder in part because of unrealistic stereotypes of beauty in the media will be happy to hear that their mental health was "one thing' and that making someone read a sad story on Facebook is "another." I think I'll stop short of hoping that people you know will die for your opinions, though.
I'm pretty sure that they did not determine from their experiments that they could control people's minds through their facebook feeds, but merely that they could make them, though some metric, feel better or worse. As for the results of this study being "obvious," there are a lot of things that seem "obvious" until we study them. The author of the linked article arguing against this seemed to think this might serve as a wake-up call for the degree we are being manipulated by invisible customization, I don't feel I'm alone in thinking this result might matter.
If we use "common sense" we can see that filtering articles out of a news feed selectively for a limited time basically makes you the reincarnation of Mengele. I think I understand.