Seems to me that the police would get a better response if they simply got a copy of what was filmed. That way the state has their evidence and the people have theirs.
Obtaining the camera (or at least the memory card) at the scene precludes any questions about the images being Photoshopped or otherwise tampered with, so I can see their rationale for wanting the cameras.
That said, without a warrant, I don't want anybody presuming he can grab my property.