The government in many Democratic countries is divide in the legislative, executive and judicial branches on the basis of kind of work they perform in terms of laying down law, implementing the law, and providing protection against unjustified law and improper implementation of law. This ensures that each of the three branches acts as a check over the other two.
The effectiveness of control of each of the three branches and of the total functioning of the government requires that this demarcation of function according to the above described criteria in relation to law is correct. The function of police clearly lies in the domain of implementation of law. Therefor it is best for the police to be considered part of the executive branch of government.
And this was an international description.
Not JUST America. But they are executive in America also.