I think the key here is that the information on the device is not observable until a search warrant is obtained.
As far as confiscating the device, the courts have held that law enforcement has a duty to gather evidence. If someone is recording an incident, the recording could hold information relating to that person’s guilt or innocence.
If the device is confiscated for evidence, a receipt must be given and the device logged into property and evidence before the end of shift. This is the policy in California and Arizona.
By confiscating the device, it prevents the evidence from being photo shopped or altered.
For the cop haters, please don’t insult the intelligent people on this forum with stupid questions like “What’s to prevent the jack booted thugs from altering the evidence?”
That's not a stupid question though — remember the parking-lot video-tapes that were taken as evidence when that Army guy was shot. (NV Costco, IIRC)
So you think it's OK to grab bystanders' cell phones?