Evidence is evidence
So, the police seize evidence of an assault (one human being having their human rights violated by another) and this is a Bad Thing? Let's not forget this could be key evidence in securing a conviction and would be questioned by the defence if it was allowed to leave the scene (where it could be edited/altered).
No journalist I've ever met (which numbers in the hundreds) would ever give up their camera for the good of another human being when they have an editor to report to later that day, so I'd say the police having that power is vital.
Mine's the coat with 'media impartiality?' written on it.