So it is possible to legally imprison someone and it still be wrong morally and in terms of the spirit of the law.

In that sense you can enslave legally while wrongly imprisoning or targeting a specific group. ie: the current drug laws allow for difference in sentencing for selling the same drug in different forms. Though it is "legal", I believe that it is wrongly targeting specific people.

wasn't it King that said that unjust laws should not be followed?