If a law is obviously ridiculous, then juries aren't *supposed* to find someone not guilty because they disagree with it, if all the elements have been met beyond a reasonable doubt. But enforcing this is virtually impossible. So if someone would stick me in a time machine and parachute me into a jury box in a fugitive slave trial, I could dig up some reasonable doubt somewhere, keep anyone from going to jail, and no one would be able to do anything about it.
And the precious Law would stand.