There was only one round of Nuremberg Trials. There were subsequent Nuremberg tribunals conducted in the same courthouse, but these were a US only effort, and not what people are referring to when they say "the Nuremberg Trials"
Those subsequent tribunals were still quite limited, with only about 1600 people being tried. For context, there were over a million members of the SS.
Further, the punishment for most of those convicted in these tribunals was forced labor, so had Von Braun been tried and convicted at that time, odds are he still would have most likely wound up working for the US developing their rockets. So the claim that the US shielded him doesn't really make sense.
The initial ones did, but there were subsequent ones targeting entire organizations--one of which was the SS, to which Von Braun belonged.