This isn't the world's best coding, but it works on a 7.0 site (might work on 7.1, haven't tested it).
Add this to the page's header injection, or to the site-wide injection (for all pages).
It should auto-populate the country field.
Note:
it will only find the first instance of a country field, not multiple forms per page
if it doesn't find a country field it will continuously run in the background... in most cases that is not a big deal, but it isn't good form. So probably best to only add this code to pages that have an Address field on them.
Hope that helps someone.
<script>
function fillCountryField() {
var field = document.querySelector('.field.country .field-element');
if (field) {
field.value = 'United States';
} else {
setTimeout(fillCountryField, 500);
}
}
fillCountryField();
</script>