You know, when I went through my teacher screening, they never gave me a psychogical evaluation. All they did was run a background check and take my finger-prints.
It's kind of both unethical and illegal to discriminate against the mentally ill. Yes, someone could have psychopathic traits, but it doesn't mean that they are a psychopath. To actually be a psychopath, they have to commit psychopathic acts--as in go around killing people or have some other criminal record. Sure you could find out that a CEO shares a personality similar to a convicted psychopath, but you can't punish that CEO for something he or she hasn't done. Just being unempathetic, callous, and a overall jerk isn't illegal, and it doesn't mean that person has to be unethical and corrupt.
A lot of business's do run personality tests on canidates for positions. Mostly these are to see how well that inidividual will fit in with the existing workers. But personality tests have a lot of flaws and large percentages in error, and can often be easily fooled. Plus, a person personality changes throughout their lifetime. Basicly, once someone has a psychopathic personality, it doesn't mean they have always had that personality or that they always will have that personality.
The study can't affirmitively say that the CEOs studied had their psychopathic tendencies before they got the CEO position and that they didn't develop them while in the position.