Where have you heard the US government is blocking the app? The gigantic security issues have been because the US government doesn't block apps like this.

It can forbid government workers from using it on government-provided devices. This is sensible due to the capabilities for arbitrary code execution and the full permissions to the device the app requires of the user. Amazon has done that with their employees as well. But the US government doesn't have a Great Firewall. Even if they wanted to, they couldn't ban or block it.

Edit: Apologies. I mistook the unlikelihood of anything like this being effective as reason enough for no one (let alone the President) to make statements like this. I stand corrected.