Shamelessly converting a comment into an answer, according to the EFF there are consequences for US citizens denying encription keys to CBP. Whilst US citizens cannot be denied admission into the US, their devices can be seized:
At the U.S. border. Agents may ask travelers to unlock their devices, provide their device passwords, or disclose their social media information. This presents a no-win dilemma. If a traveler complies, then the agents can scrutinize and copy their sensitive digital information. If a traveler declines, then the agents can seize their devices, subject the traveler to additional questioning and detention, and otherwise escalate the encounter.
Border agents cannot deny a U.S. citizen admission to the country. However, if a foreign visitor declines, an agent may deny them entry. If a lawful permanent resident declines, agents may raise complicated questions about their continued status as a resident.
The possibility of having your devices seized are further confirmed by this interview released on The Register
When it comes to data security, American citizens also have additional rights, although there are some important caveats.
"US citizens can't be compelled to turn over passwords," Nathan Wessler, staff attorney at the ACLU, told The Reg. "But border agents may make your life much more difficult. It does have the right to seize electronic devices and send them off to a forensics lab for tests that could take weeks or months."
Such laboratories are typically not in the airport building, and devices that are taken could be sent anywhere in the US. You can get them back – if you're willing to pay for delivery charges; There's no time limit on Uncle Sam holding them; and you're dependent on an overworked techie getting around to checking them out.