Lying to a federal employee is a felony; if you know you are answering untruthfully and the USG can prove it then you are probably going to prison.
It's like moving your private files from a device before travelling, you're not hiding anything you just didn't bring it.
However, they are hoping that people don't know that and do it anyway, even though they don't have to. Also, a lot of CBP employees probably don't understand that distinction anyway. It feels like it's all "on the phone".
Of course that distinction between data in your possession on your device and data that's hundreds or thousands of miles away might not matter if enough precedent accumulates to support forcing people to go fetch things when they're at the border. So we need to stand up for freedom from government intrusion now!
Except they are allowed to deny your entry if you don't satisfy their whims.
You should use your right to be silent rather than answer any questions of course, but that right disappears at the border.
Still, lying to a customs agent isn't a felony (at least not in the US). (If so, please cite the law). If you're not a citizen of the country you are entering, the most they can do is refuse entry. If you are a citizen, well there is where it can get complicated.
This is unfortunately far from the truth. See this recent post: http://www.wisenberglaw.com/Articles/How-to-Avoid-Going-to-J...
https://www.popehat.com/2011/12/01/reminder-oh-wont-you-plea...
False: the relevant statute, 18 USC § 1001, doesn't mention the word "oath" once, and applies to "any matter within the jurisdiction of the executive, legislative, or judicial branch of the Government of the United States".
Other forms of lying can also be illegal, however.