1) Facebook is not an advertising company, although they are trying to be (and so far have been unsuccessful).
2) Facebook was not just manipulating adverts, but instead your entire feed, which up until recently, had been largely organic (what your friends posted, you saw... all of it).
3) Facebook never has discussed the possibility of "testing" on users, and the general assumption they do, does not excuse the practice.
4) A/B testing for which color button makes people click more often is far different than only displaying emotionally charged posts/images and seeing how users react. This has reminiscence of psychological warfare tactics employed (and now largely banned) by a Vietnam era CIA.
5) Facebook acknowledged the need for prior consent via TOS implicit agreement... but only after they had already concluded the "experiment" (actually, 4 months after).
6) If another company, say Google, manipulated your inbox without your consent nor knowledge, you would likely feel strongly. Both Gmail and Facebook are operated by companies that can do whatever they wish -- but this does not make it right to do whatever they wish.