It is literally the same as ordering me to tell them how to make sense of the unreadable data.
I'm not giving them a plaintext copy of something already existing elsewhere (assuming no unencrypted backups), but producing new information for them.
And unless you used FDE with full authentication (AEAD), which almost nobody does, there's nothing that says that this particular ciphertext represents the exact plaintext I've had at any given point in time.
With XTS mode (Truecrypt) you can cut and paste together different blocks from different ciphertexts under the same key for as long as they're in the right positions. Depending on how much you know about the computer and if you've got access to multiple backups, you can splice together something that even if it doesn't contain anything illegal, it would look suspicious and incriminating.
Consider for example LE getting your full file version history of a container from Dropbox.
And revealing the keys is a kind of testimony that you have had read/write access to the drive, and that it indeed is encrypted (not just random).