If the US increases customs checks, and if the person is caught, that risks seizure of the phone and a fine starting at $300.
Customs are unlikely to find the phone unless they have some other reason to search, or unless it's in checked baggage and there are several.
Some countries do routinely scan all incoming hand luggage, but it's so long since I visited the USA I don't remember if they do.
In some countries visitors declare what they're bringing, so they don't pay tax on that so long as they take it with them when they leave.
What’s your actual point or argument here?
You seem to start off arguing that it won’t happen, and you finish by arguing that you don’t think that it should happen, i.e. if you had things your way, it’d be different.
Which is it? They aren’t the same thing.