Rubbish. If the service in question was directly accessible from a public network and its source code known and published, it would be attacked in less than 5 minutes. That is the assumption that causes millions of dollars to be spent on securing the application and the environment every year. We are not prepared to take that risk, and we secure appropriately and expensively.
> Also, whoever "entrusted" you may themselves be entirely reckless or incompetent.
That is how employment works in the wider world of Internet Widgets etc. but not in critical government regulated infrastructure like emergency services.
Yes we are unable to prove 100% that the application has no externally exploitable bugs, despite our best efforts, so we don't claim that it is and go and make it accessible to untrusted sources of input.