The police department later admitted that they rejected applicants who had IQ scores that were “too high” under the assumption that they would get “bored”

Well, this is quite a nice lie they have there.

The real reason is that if the cop is too intelligent, they will question too much of what police do.
It takes just slightly higher than average (mid-wits) to take the brainwashing that is police work.

Just smart enough to do the job, but not too intelligent to see that what they are doing is against the constitution and completely evil.