Encryption keys in a TPM? RAID configurations which depend on physical cabling setups? Storage that can't be physically removed? There are a thousand ways that taking the "hard drive" from a computer will end up losing information that would be recoverable with the whole device. It's not really reasonable to expect police departments to do IT foresics on site during a search.
That doesn't mean that the seizure is "justified", but there are no simple answers either.
Amusingly, the time police raided my stuff (well, they went after my roommate, and my stuff was in the living room and got taken along for the ride) -- they took the hard drives out of my media server. It was a software RAID-5 setup.
I imagine that they have the technology to rebuild the drives, but I have always smiled at the thought of some tech sitting there trying to rebuild this and wasting weeks of his time trying to figure out exactly how everything went. (It was actually a really terrible partitioning setup that was non-intuitive, with a "dead" partition that wasn't part of the RAID and used for reduced redundancy storage.)
That doesn't mean that the seizure is "justified", but there are no simple answers either.