Obvious question: how do they know that those parts of the brain were changed by the drug? Is this yet another correlation with no evidence at all of causation

Obvious question: how do they know that those parts of the brain were changed by the drug? Is this yet another correlation with no evidence at all of causation? Also, even if the drug does change the brain, is this necessarily bad? The implication seems to be that users have narrower horizons and enjoy fewer things. Is this meant to be a measure of quality of life? I know plenty of people whom I consider to have narrow horizons and it would be very easy for my to assume that they have a lower quality of life than me because they don't even derive any pleasure from things that I enjoy. That would be a ridiculous assumption, though.