And what if the police just think the drive holds the bank codes for all the money I embezzled, but is actually an archive of amusing cat pictures I forgot the password to a year ago?

Seems to me that the only way to reasonably apply "innocent until proven guilty" there is to only convict if they know what the encrypted contents are, and if they can already prove that beyond a reasonable doubt, why do they even need you to decrypt them for you? Conversely, if they don't know the contents, then they may well be innocent, and the password innocently forgotten.