At a time when more and more Americans are becoming aware of gross misuses of police power, filming the police, especially secretly, is one of the few ways to bring to light instances of police brutality. In order to have a state where officers are held accountable, it must be legal to film on-duty officers under any and all circumstances. Read an editorial about it in the Boston Globe:
https://www.bostonglobe.com/opinion/editorials/2016/07/07/epoliceshooting/v47TmG9fKMdeZylfeGsyIL/story.html#comments