It's a big bunch of random noise. Of course it's trivially easy to see the drive image is not like other drive images, which is all they're looking for. Go to jail until you hand over the passphrase.
Hidden volumes, though, enable you to place what theoretically appears to be a big bunch of random noise (unless you know the key) in an even bigger bunch of random noise with a legitimate, demonstrable purpose (an encrypted volume.)
Would you seriously attempt to convince US government agents that your 512 GB hard drive contains only a 256 GB filesystem because you're a random noise enthusiast?
It looks like there's 512GB of space committed, but contains less than 256GB of content. There is no protection against the hidden volume being overwritten, if the content in the explicit volume grows too large.
