Earlier quoted context omitted.
A brain wallet can be confiscated by rubber-hose cryptanalysis. It is more resistant, perhaps, to detection than cash on hand in foreign currency (though not necessarily particularly more than a foreign currency account where access information is memorized, depending on the threat profile of concern.)
Torture may be used to find out the user has bitcoin, but you can never be sure you got it all. If they give you access to n wallets, how do you know they don't have n+1? A foreign currency account is not as easy to setup as a brain wallet.
That's also true of caches of foreign currency in cash (not to mention remote accounts.)
Of course, the difficulty of the subject proving full compliance is more of a problem for the subject than the torturer.