This is what happens when they do: https://en.wikipedia.org/wiki/British_Post_Office_scandal
> Between 1999 and 2015, more than 900 subpostmasters were wrongfully convicted of theft, fraud and false accounting based on faulty Horizon data, with about 700 of these prosecutions carried out by the Post Office. Other subpostmasters were prosecuted but not convicted, forced to cover illusory shortfalls caused by Horizon with their own money, or had their contracts terminated. The court cases, criminal convictions, imprisonments, loss of livelihoods and homes, debts, and bankruptcies led to stress, illness and family breakdowns, and were linked to at least thirteen suicides.
Hundreds of innocent people went to prison, were financially ruined, thirteen took their own life, because a well-funded entity found it easier to privately prosecute its contractors than to question its software vendor. And the court system let them do it.
https://uk.finance.yahoo.com/news/fujitsu-allowed-bid-govern...
Also, it's important to note that Fujitsu didn't prosecute anyone, they just lied to the Post Office, lied to subpostmasters, remotely altered evidence, gave false testimony in court, etc. It was the Post Office itself that prosecuted its own subpostmasters.
> Second, this Article demonstrates that the government monopoly on prosecution is an ideology rather than a rule. As it turns out, the government has never had total control over criminal law enforcement—not even exclusive authority to charge someone with a crime. While some courts prohibited private prosecution in the nineteenth century, others refused to abolish the practice. Public prosecutions of felonies and federal offenses became the norm in the progressive era, but private prosecution persisted in various forms. Particularly in “lowly” municipal courts, private individuals retained the ability to file and litigate criminal cases.
-- The Past and Persistence of Private Prosecution, https://papers.ssrn.com/sol3/papers.cfm?abstract_id=5210126
It was less bizarre when they were all private companies, but now many are owned by the government!
Some of them have successfully prosecuted people over a £0 difference in fare
They can then do one of two things. They can either continue running it if it’s in the interests of justice to do so (public interest, reasonable prospects of success etc), or they can shut it down, as they tend to do for sovereign citizen nonsense and the like.