Many people hate cops because of the 'pig' stereotype. There certainly are many who fit this; pushy, angry, forceful, et cetera. While it is important to be able to keep track of someone, it's not right to resort to extreme measures for bizarre reasons of 'safety' when there is no direct threat present. Also, as described before, many people hate cops because they frequently waste peoples' time with bureaucratic nonsense instead of truly protecting the people from dangerous criminals. However, this is more related to bad laws and flaws in a greedy and shortsighted system than the officers themselves.
Ex. 1: The police had a typical day; they gave five hundred tickets to irritated motorists, and arrested some kids for underage drinking at a party. (common)
Ex. 2: Wow, that police officer really saved our life! It really seemed as though that guy was going to kill us! (rare)
They are constantly harassed by the lowest form of life on this planet, yet go out there day after day protecting your sorry asses from people who are sick and tired of your shit, and who if it wasn't for the Police, would be in the process of killing you, if they hadn't already.