The Fifth Amendment does not protect the password (it's just a sequence of characters); the amendment protects the "testimonial" aspect that you knew that particular sequence of characters was significant. Once that fact is entered into evidence through some other means, the Fifth Amendment's "due process" requirements have been satisfied.
That interpretation is absurd; all of ones testimony is "just a sequence of characters" yet one is protected from having to provide it by the Fifth Amendment. Further this is not a due process issue, due process requirements have been met prior to the confiscation of the hard disks and other items. It is only after which they have been taken that the investigators determine they cannot make anything ineligible from the data which they have lawfully taken. To demand that the suspect interprets the data (or provides a key by which another individual could interpret it) for them in such a way as to potentially incriminate himself is outside the scope of their authority. This is specifically what is protected by the applicable portion of the fifth amendment.
The issue people are having with this is related specifically to their understanding of data on a hard disk. Because it is not tangible in the same way as stack of photos people have a hard time determining how to apply past precedent. Being required to provide a key which allows for the understanding of data is the equivalent of a defendant in a murder trial being required to state where the presumptive body is buried. Remember, the suspect has complied with the warranted requests for the hard disks themselves, he is under no obligation to help the investigators make sense of what was taken. If the encryption key was written on a piece of paper and it was discovered by the police there would no longer be any issue, in the same way as if a map of where the body was buried had been found.