I wonder how many of those who run for public office would willingly present results of such a scan... I would bet money that that particular demographic would show an interesting trend.
I don't doubt that there may be a trend, but to me it sounds very unfair to allow people's immutable biological characteristics to enter into an adversarial debate about how they might act.
Also, the mechanics of how macro-scale neurology affects a person's psychology and sociology is still poorly understood, which makes the data especially easy to abuse. More than anything, I think the researcher's findings highlight this point; he was a counterexample to the theory that a diminished prefrontal cortex implies anti-social behaviour, showing the phenomenon to be more nuanced.
Because it is prejudicial, which is unfair because it fails to take into account the person's individuality. Such traits do not simply equate to behaviour, the relationship is complex, incomplete and the mechanics not understood. A person's membership of a class which they neither chose to be in, nor can choose to remove themselves from (in this case a genetic brain anomaly), can not infer a fact about their mind or behaviour.
I don't doubt that while some "psychopaths" will be detectable ahead of time, and maybe mitigated (whether or not that's ethical), there may be many with "correct" views read as possibly psychopathic that will get strawmanned into oblivion due to their diagnosis.
I'm not so sure about politicians but there have been studies that showed that there are more psychopaths as CEOs and people at the top of businesses. Politicians in reality do a lot of soft people work - more so than a business man would - and they are often more motivated by and need empathy for their job.
Psychopaths do have people skills, if only as superficial, learned behavior. And they have excellent intuition, and perhaps even empathy, of a sort (in that they do seem to have a feel for what others might be going for) for zero-sum, win-lose situations, like politicking. This does come at a cost, of course: they lack pro-social emotions and the ordinary empathy that relates to those; hence, they tend to fail at things like creative negotiation, adaptation and compromise (that are critical to success in win-win scenarios), and to also lack traits like intellectual and social curiosity, openness to experience, a sense for human achievement, for culture and the arts, etc. Maybe even a sense of humour! All-in-all, by and large, they are not good policy-makers - they don't exert what Bernard Crick (a widely-quoted political scientist) called the "political virtues".
Also, the mechanics of how macro-scale neurology affects a person's psychology and sociology is still poorly understood, which makes the data especially easy to abuse. More than anything, I think the researcher's findings highlight this point; he was a counterexample to the theory that a diminished prefrontal cortex implies anti-social behaviour, showing the phenomenon to be more nuanced.