From Gizmodo:

The legal justification for arresting the “shooter” rests on existing wiretapping or eavesdropping laws, with statutes against obstructing law enforcement sometimes cited. Illinois, Massachusetts, and Maryland are among the 12 states in which all parties must consent for a recording to be legal unless, as with TV news crews, it is obvious to all that recording is underway. Since the police do not consent, the camera-wielder can be arrested. Most all-party-consent states also include an exception for recording in public places where “no expectation of privacy exists” (Illinois does not) but in practice this exception is not being recognized.