Why do so many police officers act like filming them is against the law?

Most of the cops I know and have met actively encourage people to film, however they also don't want footage of their actions being taken out of context. Example: Man, who happens to have dark skin color, robs a store, shoots 4 people, steals a car, drags police on a car chase for 12 miles, before crashing and being dragged out of the car by police officers who use a lot of force to stop the mad man from hurting anyone else. Film shows police drag a black man out of a crashed car and savagely beat him. CNN, FOXnews, MSNBC, ect all run a story about how cops drag black people out of cars and beat them, playing the same video over and over again.