Wouldn’t the officer be destroying evidence seizing by the cameras?
The simple act of taking the camera would not destroy evidence. In a perfect world it makes perfect sense for an officer to protect anything that might relate to evidence of a crime, including a camera or memory cards, etc. We don't live in a perfect world though, and frequently those at the scene of a crime or emergency are not thinking all that clearly, especially about the long term effects of what they are doing.
Personally, if I had pictures important to the fair investigation of an incident I would not mind sharing them with anyone involved. The down side is that I take the pictures as part of my job and doing something with them that makes them unavailable for my employer is not fair to my employer. If I surrender my film or memory cards at the scene then it could months or years before I get it back, if ever.
Lots of people at the scene have their own personal reasons for not wanting pictures to be taken of an event. Truth is, those don't really impact on a shooter's legal right to take pictures. But reality often has a way of messing with what is ‘right’! I have had family members take my camera from me and threaten to beat me to death with it. At that point there is no reasoning with them and it's better just to quietly walk away.
I have been prevented from taking pictures at a number of scenes where the pictures would have been very helpful to a wronged party.