Pursuant to CBP’s border search authority explained above, when presenting their effects for inspection, all travelers are obligated to present their electronic devices and the information resident on the device in a condition that allows for the examination of the device and its contents. If the electronic device cannot be inspected because it is protected by a passcode or encryption or other security mechanism, that device may be subject to exclusion, detention, or other appropriate action or disposition. Additionally, the traveler may face longer processing times to allow for CBP to access the contents of the device.

Taken directly from CBP’s website; regardless of any citizenship, or nationality, they can refuse to let you reenter the country until your device is searched. Period. I don’t know where this idea that “I’m an American, they can’t refuse to let me back into my own country!” narrative came from, but it is entirely fictitious. There are dozens of reasons for you to be refused reentry.