Straight from the ACLU:

Do I have to provide my laptop passwords or unlock my mobile phone for law enforcement officers at the border?

Customs officers have sometimes asked travelers to provide their laptop passwords or unlock their mobile phones when they are entering or leaving the United States. Your legal status in the country may inform what you decide to do if you’re asked for a password to unlock your device. U.S. citizens cannot be denied entry to the United States for refusing to provide passwords or unlocking devices. Refusal to do so might lead to delay, additional questioning, and/or officers seizing your device for further inspection. The same should be true for those who have previously been admitted to the U.S. as lawful permanent residents and have maintained their status — their green cards can’t be revoked without a hearing before an immigration judge. [...]

The reason why you have 'heard this idea' is because it's true :P

https://www.aclu.org/know-your-rights/what-do-when-encountering-law-enforcement-airports-and-other-ports-entry-us?hl=en-US#do-i-have-to-provide-my-laptop-passwords-or-unlock-my-mobile-phone-for-law-enforcement-officers-at-the-border