You're missing that broadcasters transmit to a select geographical location, so they can still sell the transmission to cable and satellite networks. By making it cheaper to watch those channels outside of the areas covered by OTA broadcasts, they're making it easier for current cable subscribers to "cut the cord".
Aereo makes new users sign up with a credit card linked to a zip code within the OTA broadcast range of the metros that they operate in. The users then get the channels that are available in that area. See https://www.aereo.com/channels
Theoretically Aereo makes it more expensive to watch these channels(but easier and more reliable) than a one time purchase of your own antenna.
