The fact that behavior remains the same means they don't feel they'll be punished when found out, which isn't exactly an unfounded assumption.
Or there isn't the misbehavior you think there is.
Or, since even the cops without cameras knew they were being researched and would be more carefully tracked and evaluated than usual, they too behaved "better". IOW, the study was not "blind" in either direction, so it was a lousy invalid test of the hypothesis.
Then there are the major sources of measurement error in most of their outcome measures. The "use of force" outcome relied entirely upon the officers own filed reports.
The other major outcome was "citizen complaints". What % of citizens mistreated by cops ever file a formal complaint? Given how ineffectual such filings are likely to be, and given very reasonable fear of retribution by the officers, that % is likely minuscule. Thus, the number of complaints per study condition is likely to have little correlation to actual instances of abuse and misconduct.
In fact, those arrested would know if their officer had a camera, because that would be part of the evidence. If you know the officer has a camera, you would be far more likely to file a complaint because at least you might have some evidence and get some justice for it. Which means that the lack of difference in number of complaints is expected if the cops with cameras were objectively better behaved, but they got more complaints when they did engage in misconduct because their was possible evidence to support the complaint. In contrast, if the cameras had no impact of objective behavior of the cops, then cops with cameras would have received more complaints.
Finally, there seems to be a truly fatal flaw to the study. Half the cops in the same districts and on the same beat got cameras or not at random. That means that every cop interaction with more than 1 officer likely had at least one officer with a camera. And of course every cop would know that is the case. So, basically there were body cameras at the over-whelming majority of arrests and stops, and all that actually varied was which officers uniform the camera was attached to.