The sentiment that ex post facto laws are against natural rights is so strong in the United States, that few, if any, of the State constitutions have failed to proscribe them. The federal constitution indeed interdicts them in criminal cases only; but they are equally unjust in civil as in criminal cases.
—Thomas Jefferson. Letter to Isaac McPherson, August 13, 1813.
Retrospective laws—that is, laws passed subsequent to the act they penalize—violate the rule of law. This is because the rule of law requires that if you penalize someone for their behavior, they should at least have the chance to avoid the penalty by curbing their behavior. By extension, before new laws come into effect, a lengthy period should be granted to allow individuals and businesses to adjust their affairs to the new circumstances without undue loss.