When it became quite clear the two policemen weren't going to be able to intimidate the person into stopping filming, they walked away. Surely the police, of all people, should know the law and what they may or may not do legitimately? No doubt there will be cases when it will be a fine judgement whether an offence is being committed, but this is not one of those cases. It is quite clearly an attempt by the police to take the law into their own hands with no justification whatsoever. See what you think:
It requires a certain amount of courage, and a reasonable understanding of the law, to stand up to authority when challenged. It is necessary sometimes for citizens to make very clear that actions by figures of authority which go beyond lawful limits will not be tolerated.
(thru the devil's kitchen in turn through Tom Paine of The Last Ditch)