|this study appears not to even try to make a claim about cause and effect

But isn't that a good choice, considering the limitations of the study? It would be very difficult to prove that a diet caused any of these effects. Things like mental health have so many potential contributing factors, from genetics, to home environment, etc, that it would be very difficult to sift through and pinpoint one single cause or even one factor that provably increased the chance of mental instability. At least, this is according to what I know of Science. It's an interesting result, however, which is why it's (slightly) news-worthy. I find it interesting also that my vegetarian friends are so offended by it; while I reposted it because I found it interesting, I certainly wasn't trying to pinpoint those individuals and say that they have lower qualities of life.

I mean, studies also show that cigarette smokers are more inclined to have depression, especially if they are women, and studies show that if you have tattoos you are more likely to have major depressive episodes in your life (or at least I've heard about studies of both) - but if a friend posted a link, I wouldn't get offended by it: I would accept it. I have heard of this. (Maybe that predispositions me to be accepting of it.) I don't feel it's a personal attack. Maybe for these friends the results hit too close to home. It's not like either the study or any of the articles say "Stop being vegetarian and you will be happier/your quality of life will improve!"