Why? Well, many cops are already after citizen journalists, attempting to pass laws to limit our surveillance. Having cameras on them is their case for taking away our cameras. But if you look into the story of Walter Scott deeper, you will realize that there already was footage, Police footage, and it did not bring about justice, and it was not shown to the public until after the citizen footage went public. Even though that police footage showed that there was no scuffle and that the subject ran from the cop.
So what will happen to us if these body cameras become ubiquitous? Hopefully not what happened to Ramsey Orta, the citizen journalist behind the footage of Eric Garner who was chocked to death by cops in broad daylight and subsequently jailed on trumped up charges after his family was harassed by cops for months and months.
Watch out the Big Brother says, you dare question our authority over our injustice? And you will pay. So why would they act with the cameras any different. If recent history has anything to tell us is that they will be used against us, not against the cops. Because he who holds the footage, is he who holds the power. And they want to take our footage.