Why Is This So Hard?

If one's research question is:  "Does X have a causal effect on Y?", there are some obvious questions that one would ask before interpreting an observed association causally.  For example, "Is there something unmeasured that affects both X and Y?".  Among the most obvious of these preliminary questions is:  "Does Y in fact cause X?"  This would be especially salient in a cross-sectional study, which is why there is so much emphasis put on the term "prospective" in reporting designs that are NOT cross-sectional. 

So if that is so painfully obvious that it would be embarassing to even mention it again, why does one read reports like this one on a daily basis?  The headline is:

    Obese teens' brains unusually susceptible to food commercials, study finds

and the summary text below the headline is:

    TV food commercials disproportionately stimulate the brains of overweight teenagers,
    including the regions that     control pleasure, taste and -- most surprisingly -- the mouth,
    suggesting they mentally simulate unhealthy eating habits that make it difficult to lose
    weight later in life.

The study conducted fMRI exams on 40 right-handed adolescents, 20 normal weight and 20 obese, so everything is cross-sectional.   

The discussion reveals an interpretation clearly in the causal direction that being obese makes your brain more susceptible to food commercials:

    Collectively, these findings suggest that higher-adiposity adolescents more strongly recruit
    oral somatomotor and gustatory regions pertinent to eating behaviors while viewing food
    commercials, in comparison with their lower-adiposity counterparts.

How is it that none of these study authors with doctoral degrees seems to have ever wondered how these children got to be obese in the first place?