As far as you know. Firstly, the study is about the onset of addiction from one single experience with the drug. People that take a drug once and decide to not do it again don't attract the same social and media attention as heavy users. Similarly, if functional users of hard drugs exist, you are unlikely to hear about them. This is one of the effects of the drug prohibition: it hides a lot of data.

More worryingly, science becomes compromised when researchers cannot attract funding to explore unpopular hypothesis.