[IsThisObvious]

The debate is partly this: forcing you to produce an encryption key /also/ testifies that the drive belongs to you (or you had access to it).

You are not necessarily obligated to testify to that fact for the police, and unless they can demonstrate that the drive belonged to you (or you had access) through some other means, the production of an encryption key is tantamount to forcing that confession.

It's much like if there were a lock on a gun they found on the street: if they can't link the gun to you already, they can't demand you turn over the combination for the lock, because knowing such a combination would be a tacit admission to knowing about the gun.