I once thought that too, until I began reading reports like this one. Which surveyed really opened my eyes...
The Measurement of Police Integrity
Corruption is extremely difficult to study in a direct, quantitative, and empirical manner. Because most incidents of corruption are never reported or recorded, official data on corruption are best regarded as measures of a police agency’s anticorruption activity, not the actual level of corruption. Even with assurances of confidentiality, police officers are un-likely to be willing to report their own or another officer’s corrupt activities.
http://www.ncjrs.gov/pdffiles1/nij/181465.pdf
Power always corrupts.