It's like finding some footage that you drove into and out of a place where there was a murdered body during the same time that the body went missing. That's circumstantial evidence you might have moved it, and it might convince a judge to issue a warrant and have the police search your residence for evidence. But if they can't find anything it's not reasonable to charge you with destruction of evidence for not producing the body, because they haven't proved beyond a reasonable doubt that you could have.

People forget passwords all the time. Sometimes the police find the phone of somebody else who left it in your car and you didn't even realize it was there, and now you think they planted it and they think you won't unlock it, and the person who knows their phone is missing would rather see you in jail than claim the phone and end up there themselves. Higher level paranoia security systems can make unused space indistinguishable from encrypted data, or send cover traffic when there is no real traffic, and there is no way to decrypt it because it's not actually encrypted data to begin with.

There is no way to prove you can't decrypt something which means it's unreasonable to demand that somebody do it when they may not be able to.