Like Sandy, I would be pruriently curious as to what constitutes "obscene" pornography sufficient to be arrested for :)
As to the specifics - For the Met. to have arrested these officers, the smart phones in question were presumably police-issue, and sharing/viewing porn on workplace computers/ phones would be a misconduct issue in most large organisations, I would have thought.
I would not say such a case indicates that the police are "out of control", but it does point to management problems and a culture of elitism within closed "elite" groups, such as the Diplomatic Protection Group, although I am assuming here that the DPG is essentially a tight-knit group of officers, rather than just your average bobby being drafted in to serve for a period of time.
Hard on the heels of plebgate though, it does seem careless - although maybe this issue has arisen because the Met has been more closely scrutinising these particular officers in the light of the allegations over the conduct of serving police officers within the group rather than being an example of a more endemic, general malaise.