I'd disagree, you taking your phone through customs is indeed legal, and the border goon's seizure of you phone (by force if they prefer) and demands for your password on penalty of imprisonment is also legal. So this is not a legal issue, everyone is acting within the law. One could argue that it is a political issue, that the law should be changed (and I'd agree), but that's a different question.

That’s the point: there are multiple laws involved, court cases trying to establish where things like rights to privacy end, etc. – none of those are technical issues.