I swear I don't mean this in a snarky way: I don't quite understand how, functionally, interpreting the law is all that different from creating it. Deciding to interpret a law one way or the other (because of the ambiguity of language and intent behind laws) has impacts on people's lives, the freedoms they have, what rules they are subject to, etc.

Their job is to apply the Constitution, and in this case the Constitutional principle is quite clear (so clear that it led to a 9-0 decision). Specifically, the "takings clause" of the Fifth Amendment: "... nor shall private property be taken for public use, without just compensation."