You're confusing yourself with irrelevant analogies. You can say, "you may come into my house only if you give me your unlocked phone," and an immigration official can't assault you because there are certain protections granted to foreigners against being randomly assaulted. It's also not hard to imagine them NOT being legally bound not to search people's phones, and if you're trying to say someone's breaking the law here then it's your burden of proof.
Of course I can say that. I can say "you can't come into my house if you're black" too. The point is that it's unethical. It would be unethical for me to search your phone before you entered my house, too. This is not complicated, I'm not sure why you're having trouble understanding it.
