You're coming at this from the perspective of "it's weird to change the rules after the fact" which is reasonable but not how stare decisis works. The principle is that once something is decided once, it should be decided the same for all other parties litigating the issue unless and until the precedent is overturned. If something hasn't yet been litigated, it is still an open question.

It would be weird if, for example, the Supreme Court said that gay marriage is constitutionally protected but all the existing laws banning it could stand because they were enacted earlier.