https://www.federalregister.gov/documents/2008/04/18/E8-8339...

We’ve been doing this for 20 years. It’s the only way to be sure when somebody turns up at the border and gets in trouble a second time.

This guy showed up at the border, said he’s here to protest the government, which got him detained, which triggered this mandatory sample collection rule.

If you decide you will visit a foreign country and tell them you’re there to protest their government, you should probably research ahead of time what will happen to you.