You guys are looking at the wrong thing. Airports have their own set of bullshit rules. According to the government choosing to fly automatically implies consent to be searched. Domestic flights have more protections, but international flights basically wave your right to privacy under the 4th.

They only require explicit consent or probable cause if they want to do an advanced or forensic search of your phone. You can deny the search if you're a citizen, but if you choose that option they can take your phone into custody for several weeks to attempt to crack it if they choose to do so.

That Probable Cause standard requires a reasonable and articulable suspicion of wrong doing to exist too. Merely crossing a boarder alone doesn't give rise to that.

Of course they just deny you entry at that point...