Although the convictions may have been or may still be overturned, the newspapers would have been reporting the findings of the court at the time and which were not their responsibilty. It's heart-breaking for the poor souls that suffered from the ineptitude and viciousness of the Post Office but newspapers would be open to libel cases every time any conviction of any kind reported by them was overturned across the board.
By the way, although I watched the TV drama about it (and when I was doing something else at the time and may have missed some fine detail) how did the convictions take place without the Post Office presenting to the court a complete breakdown of the supposed takings, which could be audited in detail?
I did listen to the podcast a while ago but my memory of the detail on this front is hazy too.