Except this isn't an experiment, where they control the conditions. It's monitoring natural behavior. Just saying "placebo" makes no sense for this kind of study. That'd be like asking for a placebo in a national poll.

The kind of thing you're talking about would be an experiment, where they bring people in and use various measures to quantify various types of intelligence after smoking marijuana. In that situation yes you could use a placebo as one group. But even then it's not always appropriate, and just skimming an article and dismissing it by saying "no placebo!" makes no sense. Not all experiments do use or should use a placebo as part of their methodology, and this isn't even an experiment.