But that's essentially "breaking into your phone" against your will.

Right. But they're allowed to do that. When you cross the border, they're allowed to search your stuff. And if you don't open things up for them, they can force open your suitcase lock, they can pop the trunk on your car, they can pat you down and search your person - and you can't stop them. The protection isn't from things that are "against your will," it's from being "a witness" against yourself. It's a testimonial protection, not a protection from having your stuff broken into. The latter is the Fourth Amendment, and it generally doesn't restrict the government from searching stuff you're trying to carry across the border into the country. No probable cause, or even "reasonable suspicion," required. They get to search stuff crossing the border.