There are instances of allied war crimes that went unpunished, you win you're right, you lose you're in the dock.
To be fair, I think there were probably quite a few Nazi war crimes that also slipped though the net amid the general barbarity.
One of the reasons the prosecution at Nuremberg succeeded was the meticulous record keeping in the concentration camps. Ad hoc atrocities tend to be harder to prove and take much longer to do so (look at the former Yugoslavia).
The occupying powers did more than turn a blind eye to Nazi atrocities and systematic brutalisation of the population - they actively recruited the Nazi state machinery to run Germany post war. Yeah, the figurehead politicians and architects of the Nazi regime were put on trial - but the nuts and bolt bureaucrats were co-opted by the occupying powers to continue running things.
The same people who ran the railways to the death camps, the logistics of mass murder, the 'banality of evil' as Hannah Arendt named it - ran Germany post war. There was a programe of de-nazification but practical necessity quite often trumped morality.
Not to mention the wholesale transfer of the V2 rocket programme (ran on slave labour - expected life span of a slave - 6 weeks) to NASA/ Strategic Missile Air Command. In a very real sense if it wasn't for the Nazis we might not have made it to the moon by the 1960s! (if we actually did - for the conspiricy theorists)
History, or what we understand as 'popular history' is quite often a reflection of the here and now.
I'll give you an example. Academic researchers researching dusty archives are all after one thing; and that is funding to enable them to do academic research in dusty archives. Where does the money come from? - well a variety of sources but in times of recession it is really hard to come by, believe me. As a researcher you will encounter gatekeepers to this funding and they will quite often narrow the parameters of your research to reflect their own interests... so, do you see where we are going with this?
If you're thinking 'He (and it usually is a 'he') who pays the piper calls the tune' - you're not too far from the truth.
I'll give you an example
In 2014 Mr Cameron in the midst of the worst recession since the Great Depression, in his wisdom is spending £50 million of your cash on events to commemorate the 'Great War' or WW1 as it's now called. From a personal perspective I'm of the opinion that this is a war that should never be forgotten and it could be money well spent - but it should be spent for the right reasons. I'm quite sure that this money will be spent in evoking a 'we were all in this together' type national unity version of the history of the first world war - there certainly will be little, if any, discussion of crtical narrative of the history of WW1.
So, in general, you get the popular version of history the powerful pay for - real history is often done by learned amateurs with little money but lots of love for the subject.