Given a set of facts, it isn't always clear if something is legal or not. Hence, case law, and appeals and so on. Was obamacare illegal? It wasn't clear. A judgement has been made at this point, but to suggest it was an "objective" decision wouldn't be accurate. The supreme court judges could have just "decided" it was illegal if that was their opinion. Several did. The very existence of the supreme courts highlights the fact that the law is a subjective thing, there isn't some nice big leather bound book which you can just consult for every single situation and go "yep, ok, that's legal".