[stinkytaco]

Because police engage in dangerous situations by their very nature. Following them around will likely put followers in unsafe situations as well. Following them and engaging in antagonistic dialog will detract from their ability to do their job. Imagine trying to asses an area before entering a building to deal with a domestic (likely one of the most dangerous things an officer deals with on a regular basis) when someone is antagonizing you.

Filming should be legal. If there is an interaction occurring, police should have the expectation of being filmed. They should be required to wear body cameras. They should be accountable. But what this guy was doing seems to slip over the line into obstruction.