I don't even understand the logic behind this(maybe because there isn't any). Unfortunately, police mistakes happen, you won't become a professional just by wearing a uniform. And braindead laws make their job even more difficult.
A question though, what grades/school do people need in the USA to become a cop?
At this point, it is more than a few mistakes. The whole institution is corrupt first of all because it is funded under threat of violence (via taxation) by extortion and its "services" forced upon non-consenting "civilians."
This is why all state police around the world are morally illegitimate.
That said, US cops are arguably the worst, and departments actually are allowed to refuse employment to applicants who score too high on IQ tests.