Is there a way with one key press to set all default regions to United States?

This is an excellent suggestion!

To change Default Regions, you have to look under Tools, Default PTP Regions and Default PMP Regions, and set each band's default region, one by one. This can take a few minutes.

Including a button to set the default region for all bands is a little tricky, since the bands' region names vary (FCC, United States, FCC Lower, FCC Middle, FCC Upper, etc.)

We will consider putting this into a future release.

Thank you for your suggestion!