For most people, a phone contains:

1. An itemized list of every account they have and their passwords and any 2FA they've used.

2. Multiple forms of payment.

3. A photo gallery that, in many (if not most) cases, will have private content.

4. The contact information of everyone they know, who have not consented to having that information distributed.

I find the entire idea that it's acceptable for any barely-educated border agent having a power trip with no probable cause to demand this under the duress of rejection, repugnant.

Especially in a country where police officers are occasionally allowed to "have sex" with people they apprehend - it seems like there's nothing that actually stops a CBP agent from just stopping any attractive woman they happen to see and demanding their phone so they can find nudes.

Say no? Get deported, no legal recourse.