Body cams are meant make the police accountable to the people. Mass surveillance gives power to the government to track and squash dissent.
It's the same reason we demand transparency from governments, and privacy for the people.
There are smaller aerial drones that could be used, which would capture the same relevant footage, without the potentially miles-wide dragnet. That's my main objection here: innocent people shouldn't have to give up their rights because the government can justify it. A good justice system would do their best get the minimum amount of information necessary, in order to protect the innocent.