I suspect that the people they're trying to catch by doing these searches (pedophiles, illegal workers) are generally too stupid or lazy to bother hiding their data. At least, that seems to be the case when watching Border Security. They do actually catch a lot of people this way.
So you would catch the lazy criminals, and maybe even victims of such act, where things were done on their devices without their knowledge... I've read recently about new kind of software, that encrypts your data without your knowledge and then asks you for money to unlock it - wondering how this would play in courts...
It sounds like an encryption suite that had had an included "crytpolocker" gui element would be a hilarious method plausible deniability, and at least make this asinine habit harder. Just randomly generate the BT address so some strangers get paid if the border agents want to try to unlock something they have confiscated.
