Our Police have a huge weight around their necks which is put there by the fear of the media and the human rights gone mad elements. This was very evident in their performance in the riots.
While it was unnecessary for PC Harwood to push the guy out of the way, you have to take into account the huge stress an ordinary policeman is experiencing in violent protests.
Although, I can see why Tomlinson was pushed, he was moving incredibly slowly and not exactly responding to the police when told to get out of the way.