The difference is not because the mind is different, but because we cannot infer its content otherwise reliably. (That's the problem with polygraphs, they are extremely unreliable relative to the importance of their purpose.)
If it were possible to do it with some sort of scan (reliably!), eventually courts would order those scans.
But that's kind of the point: the idea that sanctity of one's thoughts is not ensured, that really it's just a technological limitation, should be extremely troubling.
If it were possible to do it with some sort of scan (reliably!), eventually courts would order those scans.