Fujitsu/ICL won the contract to develop and run Horizon. They got a commission on every EFTPOS sale. They paid for all the computers, all the network setup, all the staff training. They literally ran the helpline. If you were a sub-postmaster and had a problem with Horizon, you called Fujitsu.
It was Fujitsu that then told you that the bug you found in Horizon wasn't a bug and nobody else was experiencing it, at exactly the same time their internal IT tickets had fully documented the bug and their staff were trying to patch up that bug before it happened to anyone else.
Fujitsu also claimed, in many court cases, that they had no remote access to Horizon. But they did. They also let engineers use it, and push one-off code fixes, to "fix-up" known errors that had been made in ledgers on the computer in your Post Office, so there was no source of truth anywhere in the system. If courts had known this, almost every Post Office private prosecution would have been thrown in the bin for unreliable evidence. Instead, courts ran on the belief that computers were like calculators, and can be assumed to be reliable unless proven faulty.
It was Fujitsu not volunteering this fact, and indeed barristers coaching Fujitsu expert witnesses on what to say and what not to reveal, ignoring procedural rules that the barristers knew had to be followed that say you have to reveal pertinent facts to the defence.
Fujitsu were in it up to their necks along with the Post Office. They made material gains by denying bugs existed, denying they had remote access, falsely claiming their system was reliable, and having their staff perjure themselves in prosecutions brought by the Post Office.
Without Fujitsu's complicity and mendacity, the Post Office might not have succeeded in prosecuting anyone - and of course, without the phantom losses caused by their broken software, they'd have no cases to prosecute.
