My take on it, is that some of them got caught up in the horrendous culture at the time and due to the perception of peer pressure, either went along with what was going or turned a blind eye.
There wouldn`t appear to have been any option of whistle blowing either and even if there was, the main body of the wrong doers, was that powerful, I`m pretty sure they`d have buried anyone who attempted to whistle blow.
When Vennells gets on then stand, if I was the prosecution, I`d be really driving home the incredible contradictions between Vennells alleged Christianity and the criminal conspiracy between her and the higher ups in the Post Office.