Earlier quoted context omitted.
For non-citizens, yes. But they cannot deny entry for US citizens, and they cannot detain citizens indefinitely.
I suppose an interesting form of DoS protest would be to have a large group of people refuse to unlock their phones and take up all of the space in the customs holding cells.
If you have any doubts about that, you should visit a US-Canada border crossing. The overwhelming majority of travelers are not subjected to through inspection.