We are not talking of their personal phones. When you are issued a phone for work, there is no doubt your employer can dictate what gets installed on it and what not; exactly the same as in your office PC. They can force you to use a corporate wallpaper, theme, sounds, browser, everything. There is no doubt about it. It is their phone, not yours.
The real issue here seems to be that the app security is not great, and does a lot of calls to third party APIs, which is a really bad idea on a government issued phone which contains extremely sensitive data and connects to hardened internal networks.
The rules of governments are not supposed to be the same as the rules of corporations. This is simply an effort to put propaganda in the hands of every government employee. Our norms used to prevent government propaganda channels.