The devil is ALWAYS in the details.
I would think it's because that's what the researchers chose to study. All studies need to study something, and most could have multiple subjects. Studies often find both related and unrelated information leading to other studies, they's why they are conducted.
Because the study simply involved those who had had breast implants. That was the population they studied. That's what statistics does. It studies certain populations for certain statistical characteristics. You can "say" that "people who do not like their bodies have more emotional problems.....", but you can study certain populations to see if there is statistical evidence of what you say. The study is just evidence, that's all. It looks like any other normal research to me from the article information given.
Some people don't like the results of statistical studies of populations for their own reasons, but if the research was done properly and ethically and interpreted properly, then what's the legitimate complaint?
Well, the simple answer is because they found a correlation between breast implants and suicide, and are reporting on it. Do you think they should have mentioned a propensity for dog-kicking, just to keep things even?
The article is very careful to state that the correlation is probably due to pre-existing psychological problems.