Misleading headline. Faithless electors CAN go rogue if election time rolls around and their State law does not explicitly forbid it. The Faithless Elector wiki page has a map and those are the states in grey. Some States impose fines (lame) but most subject defiant electors to recall and replacement (their vote disqualified).

The ruling is a unanimous clarification that says is IF the State imposes rules of conduct for electors (the orange states on the map), they cannot act like individuals who have been given some new comic book president electing super power and cast votes that defy their home states. Or they can, but they'll just get rubbed out and replaced.