Clearly Aereo could sell antennas that people install. And they could also install those antennas. Presumably they could rent the antenna and the cable to them instead of selling it. So at what point does it cross the line into a service that is illegal? When they use a signal booster? When they convert it to a digital signal? When my neighbor signs up too and they use a common cable along the way?
I suspect the real problem is that the tech has just changed too much; at some point original intent stops making sense when the circumstances change enough. If we had a well-run legislative branch, presumably they would have noticed this problem a few years back and patched it. Or they would have delegated the problem to a regulator whose job it was to keep on top of things. But that's not the world we live in.