As a former DC patrolman, I can tell you that protests of all stripes in DC are routinely videotaped, regardless of whether the officers on the front lines have their body cams on. This is to identify violent protesters within the ranks of their otherwise peaceful brethren.
I read this as an apparent attempt by the protest organizers, aided and abetted by the local government, to conceal the identity of the protesters. It will not work.
Making a public announcement that street cops aren't allowed to use body cams could = rioters letting down their guard a bit, allowing 'other' security cams a better look.
Personally, I hope they turn firehoses on the bastards, then let the dogs loose, then shoot anyone who gets past the dogs.