When you generalize about a population of people, you're unfairly attributing characteristics to them which weren't decided upon by them (and likely doesn't apply to all of them).
When you generalize about police, you're fairly generalizing about a group of people who have decided to be a part of that institution, have decided to behave in such a way that would not get them kicked out of the institution (ie. they haven't agitated for any significant change in almost every case), and have decided not to leave it with full knowledge and in spite of its abuses.