To elaborate on this: Ideally, encrypted data is indistinguishable from random data, otherwise known as "noise". Sensor data, radio telescope data and so forth

To elaborate on this: Ideally, encrypted data is indistinguishable from random data, otherwise known as "noise". Sensor data, radio telescope data and so forth often contain lots of that: it's just a LARGE file of bits that seem uncorrelated. No one can prove that a multi-gigabyte file of recorded data contains that, as opposed to them having renamed super-secret.tar.gz to sensor-logs.tar.gz. Since no one can tell the difference, there's a pretty reasonable fear that police could see data related to your hobby (dumping ROMs, analyzing data, etc) and say, "You need to decrypt this so that we can see that it doesn't have $(illegal stuff) in it".