"Outsourcing" the running of post offices forced individual postmasters to negotiate contracts with a national institution.
Without any of the opportunities for collective bargaining they would have had as employees of the Post Office. Nor any of the leverage that market competition would have given them if the Post Office had to compete with other upstream suppliers.
So they were easily coerced into using the Fujitsu bossware. With no freedom to replace it, or even audit its effects.
(7/?)