Proof to a criminal standard may be hard to establish, even in such obvious cases as all the officers who saw what happened to Ian Thomlinson, but did nothing at all to the person who clearly pushed him over.
You refer to that as an "obvious case" ... but it is far from that.
As far as I know we have
still not seen any footage of what happened in the 30 seconds / 50m before the push and that may well change things entirely.
If (as everyone seems to do) we assume that there was no prior interaction between Ian Tomlinson and the police line before the baton strike and push then it
is a pretty obvious case of excessive force - I cannot conceive of an explanation of why the immediate use of such force to some person simply encountered as the line moved forward was either reasonable or necessary.
But, if (as I suspect is far more likely to be the case) there
was some prior interaction - either him trying to force past the line or resisting them moving forward, or deliberately moving back excessively slowly, let alone any direct aggression - then the use of
some force could well be justified ... and so it is no longer the sort of clear cut case that you suggest.
In public disorder situations there is lots of force used on lots of people and for lots of reasons - it would be unusual for any particular use of force to be such that it was "obvious" in the same way that a similar use of force, used by an officer wandering down the High Street with a colleague on Tuesday afternoon may be "obvious".
In any organisation, especially one which regularly confronts danger, members are prone to look out for one another and whilst we can all hope that it would happen, I think it is naive to expect minor breaches of the rules or the law to be routinely and immediately formally reported, even by next level supervisors. The police are
pretty good at reporting more serious stuff most of the time (especially when it is an entirely innocent victim involved), though there have always been (and I suspect always will be) some bad exceptions. And there is also a very significant aspect of informal discipline, either by fellow officers or by first line supervisors, reading officers the riot act or making it plain that they were out of order. This is particularly so in cases where a bit too much force has been used (but where some was clearly justified) or where the victim is not a particularly pleasant individual themselves and it is considered that a formal complaint / criminal conviction would be disproportionate to the context of the incident.
Ethics, although apparently very straightforward, are a majorly difficult issue in much policing. It is not a subject which is discussed at the level of operational officers much, if at all, although it does tend to form the basis of intellectual debate at senior command level, particularly on training courses and at conferences.
(And, not wishing to miss an opportunity to do the joke, it is always worth noting that, anyway, the nearest the Metropolitan Police will get to Ethics is somewhere round about Womford ...

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