notionally, the rule was (il)legal since its enactment. rulings merely affirm the legal status of rules, and give the Court the opportunity to order remedies.

for example, if the Court ruled that the Controlled Substances Act were unconstitutional, the government would be forced to free prisoners convicted of violating it, since the law was illegitimate the whole time. but if Congress repealed the act, they'd have no such obligation, since it was still a crime when the felony was committed.