They usually fall back on the good ol' "interfering with a police investigation". And if the actually do arrest them they can add "resisting arrest".
I guess the next question would be, “If I’m videotaping you is breaking the law, shouldn’t you arrest me? If you saw a guy shoplifting, would you tell him to stop shoplifting or you’ll lock his ass up? If you saw a guy raping a woman would you tell him to stop raping her or you’ll lock him up? Where did you get your badge? At the bottom of a Crack Jack box?”