It's simple enough. If there isn't the evidence to convict the person is innocent.   "Evidence" rising from torture is not evidence, and is not admissable. That's how it's always suppose to work, and there's no justification for changing it in these cases.

Maybe some people will get acquitted who are in fact guilty. But that's always the case, and on balance its recognised that its part of the price of a tolerable system of justice.