Yea, another company similar to Aereo had been trying to get themselves declared a cable company so they could take advantage of the cable company mandated retransmission laws, and had been unsuccessful in the lower courts.
The “content providers” (which is code for the current cable companies, since the big content providers like Disney own the cable companies) argued that Aereo was acting like a cable company, so they must fall under the law governing cable companies (which means Aereo had to pay to retransmit over the air content). The Supreme Court agreed.
I bet the big cable companies are kicking themselves now. This has opened the to door not only to Aereo, but to dozens of other companies to become “internet based cable companies.”
The barrier to entry is low (since you don’t have to run any physical cable). You could literally get started by putting an antenna on top of a building and a few computers/servers in the basement.