"Because only lawyers could come up with the absurdity that you can be compelled to provide a key, but not a combination."
Actually -- this is not an absurdity. The combination could well only be known by one person. Keys, however can be copied. Duplicated. Or even Mastered.
Point : "give us the key for your safe" versus "give us the combination"
A key is a physical object, where there may be more than one. And likely are more than one. Combinations, although they can be shared could well be only in the mind of one person. I would gather that the differentiation comes from the fact that the manufacturer of said safe could well have a copy, or a master key, thus making the refusal of handing over the key a simple delaying tactic, since the prosecution could apply to have the safe opened by the manufacturer.
Not that ANY safe, once in the hands of any competent law enforcement agency would not be opened using *some* method. Key or not, combination or not. Which effectively moots the entire point of comparing "Safe key/combination" to "encrypted hard drive password"
Brute forcing a decent hard drive encryption routine, given decent salt, decent passphrase, and decent encryption routines could take several years -- even IF you have *really* good compute engines to chomp on the issue.
In most cases a couple of HOURS of work with appropriate welding tools will have that safe open.