I suppose the obvious differentiator is whether content was performed to be recorded, or incidentally filmed. 'Snuff films' and other intentional-for-filming recordings are usually illegal (or at least investigated to find the producers), while recordings of general violence aren't.

It's a touchy distinction (as any intent charge is), but it makes a certain amount of sense - one major reason for criminalizing content is to reduce demand and thereby hopefully prevent further production. Recording an accident or unrelated act of violence presumably doesn't create any incentive for more violence.