It's not clear from the article, but the debate in Germany is about local offices using WhatsApp to interact with citizens. Like people asking about garbage collection dates, or informing about school closings. Things you might as well post on a wall or send with a postcard. Anything remotely sensitve, like getting a certificate of residence, has to be done in person anyway.
You're right when it comes to government officials working with classified information. But otherwise its completely normal to subcontract IT to third parties, even (especially) in the US.
Many people on HN say over and over that Facebook is a private company and they can ban you whenever they want for any or no reason.
A company that behaves this way cannot be trusted of course. If I get banned I cannot know about closed schools and garbage collection dates? And with no way to appeal or get redress. Of course completely unacceptable.
You're right when it comes to government officials working with classified information. But otherwise its completely normal to subcontract IT to third parties, even (especially) in the US.