[rawgabbit]

Serious question. If I am re-entering the US after traveling abroad, can customs legally ask me to turn the phone back on and/or seize my phone? I am a US citizen.

Out of habit, I keep my phone off during the flight and turn it on after clearing customs.

[Analemma_]

If you are a US citizen, you legally cannot be denied re-entry into the country for any reason, including not unlocking your phone. They can make it really annoying and detain you for a while, though.

[monocasa]

They can also practically keep your phone indefinitely.

[verall]

my understanding is that they can hold you for a couple days without charges for your insubordination but as a citizen they have to let you back into the country or officially arrest you, try to get an actual warrant, etc.

[direwolf20]

they can just break the law

[rurban]

There is no law when entering the country. They can do everything they want, or making up anything they'll imagine.