The point is not what they believe, but how to avoid both breaking the law and not giving up your data, isn't it?

As long as that's legal, what they believe doesn't matter.

You are not legally obliged to keep your data on your devices all the time.

What do you mean?

You're right for non-citizens.

For citizens, no one needs to believe it. You give them your empty phone and they can't download your phone contents and contacts.

Generally, within 100 miles of a border (which includes international airports) there is not a warrant required for a search.

Which is where 99% of people live btw

They can search as much as they’re able to, but they can’t compel a citizen to supply a passcode.

You need it to look set up enough to do critical communication / tourist activities. It should look like you had to reinstall your OS right before your trip, and only had time to get the basics on there. Most people will prioritize communications, maps, and payments over anything else, with video/music/books/podcasts following that, and signing into the vast universe of social apps later if ever.