The American justice system works on the idea of precedence. Cases have ruling decisions and the interpretation of the law that comes from those decisions becomes law. It wasn't clear before the ruling because there was no precedent. Now the precedent that has been set that going forward, the supreme Court (currently politically motivated to the right) will have final say over whether or not a sitting or former president may be tried and prosecuted for decisions they made or actions they took in office. What would have been the correct thing to do with the least political implication (the supreme Court is meant to be free from political biases) would have been to define what actions are illegal according to the law. But they didn't want to define actions as legal or illegal, they want the ability to justify them making case by case judgements which give them the opportunity to push their aforementioned bias.