The private keys are usually encrypted, so the attacker would have to brute force the password, install a keylogger or use a wrench, to give some ideas.
> (9) A notice under this section shall not require the disclosure of any key which-
> (a) is intended to be used for the purpose only of generating electronic signatures; and
> (b) has not in fact been used for any other purpose.
I'm not versed in bitcoin enough to judge if wallet data protected by that password is used only for signing. This clause seems largely meant to avoid the possibility that authorities will impersonate a user after forcing him to hand over his key.