It's sad this is the default view. It's his device, his data, his life on that phone. If he had wiped the phone before the interrogation it wouldn't be a problem. How long before? A second before? A week before? But wiping the data a minute later is suddenly asking for trouble.
If you're going to be in a situation where you're in a room with some goons backed by the full power of the state, it is what it is.
Maybe because I'm not American I don't have any hangups about seeing the US government this way, but my own government is no different - you can (and people have) get stopped at Heathrow, taken to a dimly lit backroom and given a going over for hours
Edit: a bigger picture question is the difference between things that may be legally punishable and things that may cause suspicion from CBP agents, which aren't the same thing at all.
A non-citizen who arrives with a blank phone might be denied entry because it seems suspicious.