Court had previously charged Wenzday Jane for "interfering with a police officer" after she used her phone to video-record police questioning three teenagers on a playground.
First Amendment protects rights of demonstrators, including the right to record police officers performing their duties in public.
Regional police group denies access to public records, claiming to be a private entity.
ACLU details injustice and suffering, particularly for people of color, caused by military approach to low-level police work, with little public input or oversight.
Statute laying out clear rules and oversight needed in light of revelations about program now suspended by Boston Police.