Of course they should. But that's no the issue here. The issue at hand is whether or not the government can compel you to give them information.
Consider this. You have a box, buried in the Mojave desert, with some documents in it. You've memorized the gps coordinates of this box. Should they be able to compel you state those coordinates?
Sure, they could search 100s of square miles looking for your box, and they might stumble upon it. But they shouldn't be able to force your to tell them where it is.
An encryption key is the same. They can try to brute force it if they wish. They're just looking for a much smaller box in a much bigger desert.
A subpoena is a compulsory order to produce all documents related to a matter, whether the government knows they exist or not. So yes, if you believe in subpoena power, you agree that you should have to produce that box.
The Fifth Amendment, as currently interpreted, doesn't provide the protection you describe. It protects against giving self-incriminating testimony and essentially the question is whether the password is "testimony." Keep in mind that the government is not asking him to disclose it, just to type it into the computer. Here's a good discussion: http://volokh.com/posts/chain_1197670606.shtml (There seems to be a CSS problem with that page, but the text is fine.)
You have the coordinates of the dead body, or they can dig and scan each square foot of the continent. This is not a Fifth Amendment case. They don't want or care about the coordinates itself. They can't use the information you provide (the coordinates of the dead body) to incriminate you, no matter what the coordinates of the dead body is.
> US v Hubbell makes clear that the government cannot compel production of documents (except under immunity to prosecution) which it does not know, ahead of time, to exist.
Dunno if that's true (as IANAL), just quoting.