For one thing, the n sample size is too small to be confident of conclusions. Since this is a non-interventional approach, and no harm to subjects comes about, a larger study using hundreds of subjects is needed. A second criticism involves the self-statements of the subjects as to how often they smoked, which was seen to correlate with degree of "abnormality". This is likely to be wildly variable in reality and some subjects have reasons to not give correct answers, yet the amount of smoking was found to predict abnormality by degree very nicely, in a manner which seems a little too good to be true, especially in such a small study. A third criticism is that no functional description is tied to the abnormal brain structures, other than the rather large leap to the animal study and the conclusion about diminishing efficacy of natural reward. No test scores, etc. Individual brains vary a lot from one person to another. It is conceivable that the "abnormality" could even be superior in some way, if it in fact exists as a result of the THC ingestion.
My impression is that these neuroscientists (a field in which I was trained also) went into the study to prove something and By God did they ever find what they were looking for! Maybe a little too enthusiastically. We will have to wait for future larger, longitudinal, and function-measuring studies to clarify the real cause and effect involved here, and if the "abnormalities" are found reliably and even matter.
Does your critique apply to the Psychology Today study cited as well?