Here is a simple rule of thumb: Public officials should have no expectation of privacy when performing their public functions. Period. Except for some really narrow exceptions, I can't think of any justification for prosecuting people filming police officers than the officer's desire to avoid accountability.
I grew up in the South and saw the tail end of Jim Crow. This is how we are achieving the new equality -- in the future, everyone will be treated by the criminal justice system like blacks have been in the deep south.