I've just listened to the podcast linked to above - https://audioboom.com/posts/8417438-ep-34-where-did-all-the-money-go
I think MasterBeth's point above is the crucial one here, and @PerkingFaintly said the same thing in a different way:
Just as with Crapita, the DWP and DWP service-users, so too do Fujitsu, the PO and the subpostmasters & mistresses sound like a classic case of Supplier, "Customer 1" and "Customer 2". Customer 2 is the end-user, and is captive with no choice but to use whatever product or service Customer 1 procures. The supplier and Customer 1 care only about their cosy relationship with each other; Customer 2 gets screwed.
The Post Office had no incentive whatsoever to sort the problems out. They believed (wrongly, I think, as decided in one of the court cases) that their contract with the subpostmasters meant they could retrieve all losses from them, no matter how they arose. The examples given in the podcast aren't just about the Horizon system. The PO could demand subpostmasters reimburse the PO for things like this too.
- Employee of subpostmaster steals money from till, subpostmaster knows this but the Investigation Branch refuse to look into it, police say 'Nothing to do with us, the PO investigate all crimes to do with the post' and the employee has vanished along with the cash. PO say to subpostmaster 'Hand over the money, it disappeared on your watch, it's your problem'.
- The PO's central accounting office puts the wrong code on a transaction or miscounts the money sent to a sub post office or somebody there miskeys an amount. The sub post office ends up in deficit. See above for PO response.
They went to great lengths to vet subpostmasters to make sure they were honest decent people before allowing them to take on the role, but from that point on seem to have switched to the default assumption that everyone in the system was dishonest and on the take.
They also seem to have been too ignorant about computer systems to be competent to assess what Fujitsu were palming them off with. There isn't a computer system in the world that is free of software problems. How could they claim Horizon was running perfectly and expect to be believed? Why, indeed, were they believed, in court case after court case?