Android: Why are unknown devices being registered from the United States?

When you publish your app to Google Play, Google uses automated testing mechanisms to ensure your app doesn't contain malware and doesn't breach the Google Play Terms of Service.

 

To do this, your app is downloaded and installed on these test devices which are located in the US and other countries, even if your app is in "Beta" distribution mode. This is why you are seeing unknown devices being created for your app.