To my lay understanding (IANAL) it seems the legal system used to have a strong distinction between "things you have" and "things you know". To the point in the U.S. where "taking the 5th" is very much in the public consciousness as not having to say anything that could be used against you. A warrant certainly can be used to take the drive with any encrypted material on it, but forcing divulgence of the keys seems like another level entirely.