The Copyright Act is a federal statute that protects any public performance of a copyrighted work from infringing uses.
Aereo took what were undisputedly public performances of works from broadcasters to the public, intercepted them, and by a feat of what amounted to technological legerdemain turned them into what it argued were no longer public performances but, instead, individualized transmissions from its service to each of its end users as an audience of one.
In this case, the Supreme Court rejected that argument, relying heavily on the idea that it was merely interpreting a statute (the Copyright Act) that had been amended by Congress in 1976 in part to overturn a couple of Supreme Court cases that had interpreted the prior Act to permit cable-TV style transmissions of copyrighted material free of copyright restrictions. The Court held that, in doing so, Congress intended to bring cable-like services such as Aereo into the Act's sweep regardless of the particular technological ways in which it handled the copyrighted material as it re-transmitted it to its users. Treating such technology, in effect, like a black box, the Court emphasized that, regardless of its innards, this service appeared to all concerned - to broadcasters, to viewers, etc. - to be functioning just like a cable service and therefore fell squarely within the Act's coverage as Congress had intended to modify it in making the 1976 amendments.
The most fascinating part of the decision, in my view, was how the Court arrived at this result while simultaneously trying to narrow its impact so that modern technological innovations would not be hit by its shrapnel. The Court not only did so but did so emphatically. Indeed, it devoted an entire section IV (pages 15-17) of its opinion to that issue. "We agree", said the Court, "that Congress, while intending the Transmit Clause to apply broadly to cable companies and their equivalents [i.e., Aereo], did not intend to discourage or control the emergence or use of different kinds of technologies." The Court then goes on to suggest (without deciding) a number of potentially key distinctions by which things such as, e.g., cloud music services might not in any way be infringing, including the prospect of fair use or the fact that the user receiving a transmission from the service may already own the copyrighted works being transmitted. I believe this is a strong signal from the Court that lower courts are not to ham-handedly interpret copyright law to stifle innovation but are to apply it carefully to prevent its abuse.
All in all, this decision represents a guarded upholding of traditional IP protections that prevents the use of technology to enable free-wheeling use of broadcasted materials while at the same time limiting its holding to that narrow sphere. Given the technical wording of the Act, it could just as easily have gone the other way and upheld the Aereo service as nothing more than something that facilitates individual, "private" performances via a streaming technology. But that would certainly have glorified form over substance and, I think, the Court got it right in the end.