Earlier quoted context omitted.
"Can't". I do not think that word means what you think it means. Of course they can, both technically and legally. Ethical sketchiness has never been a consideration for giant corporations before, why should it be now? You are not Facebook's customer. You are their product. You have no expectation of privacy for anything you post to Facebook, whether or not it's "private".
I'm not sure why this post is getting down-voted aside from people not liking what it says (perhaps how it says it?). Nothing it stated was factually inaccurate or unjustified. You really aren't Facebook's customer - no money is exchanged for services rendered.
In short, because it doesn’t have much substance and may actually be counter-productive.