John is worried about traveling overseas and then having his mobile device or computer taken away and being forced to unlock it. Leo says it doesn't really happen all that often, though they do have the right to do it, and it hasn't been ruled on being unconstitutional just yet. He'll have to worry when they take his device into another room. Chances are, they've cloned the data or even installed something to monitor him. At that point, if he's security conscious, the only thing he can really do is replace the device to be sure he's secure.
If they're going to that level of effort, though, they're identifying him as someone to investigate, and there's really not much he can do. The ACLU is fighting it, though, and will take it to court. He has the right to protect his privacy, but then he has to decide how much inconvenience he's willing to undergo to assert them.
A way to solve this is just use a dumb phone.