US Supreme Court rules for Google over Oracle

Ruling on the narrowest available reasoning is a part of judicial restraint. If they can, judges will usually try to rule narrowly so their decision affects only the one case rather than broadly in a way that affects other cases. They can't necessarily avoid broad rulings- ruling in favor of Oracle in this case would have required them to decide that APIs were copyrightable- but when given a choice of reasons for ruling a specific way, they will usually choose the narrow grounds over the broad. This is true even of the Supreme Court, which has the power to set binding precedents and theoretically could change the law as it sees fit.

Even when looking at the details of the case, there's a clear hierarchy of reasoning. First they'll look at whether the plaintiff has the right to bring a suit and if they've chosen the right court. Then they'll look to see if their legal theory of the case is correct. Only when they've gone through that part will they bother looking at the facts.