Given that the TV broadcasters are paying for the right to broadcast (often gigantic amounts of money), I don't see a problem with preventing an Aereo from free-riding on that privilege. YMMV.
If you look back before cable TV, broadcasters transmitted their signal and the only people who could watch it were those with suitable equipment within range. Broadcasters competed to get as many people as possible to watch. Each new eyeball (preferably in pairs) meant additional revenue for them with no additional cost.
Then comes cable TV. In addition to access to new channels, it also provides the benefit of being able to watch regular broadcast channels even when you couldn't receive them over the air. That means more viewers for the broadcast stations, and thus more revenue. Why, then, should broadcast stations get paid for distributing their signal to more people, which is what they want anyway?
It's clear to me that this is what the law says, and since the Supreme Court's job is to interpret the law, it also seems clear enough to me that they made the right decision here. But it doesn't seem like a very good law.
Tech people like to treat laws as rigid rules that would be written in a formally specified language if only legislators were capable of such a thing. But they're not, and by design.
It doesn't make sense for a law to be written such that receiving on one antenna and sending the result to your users is illegal, but receiving one an antenna farm all located in the same spot and sending the result to your users is legal.
Do we really want to live in a country where the obvious solution is illegal but an insane workaround is legal? I don't want a precedent set where it becomes standard for laws to be bypassed with expensive and technically pointless workarounds. If we want to allow companies to receive TV broadcasts and stream them over the internet, we should do so. If we don't, we shouldn't. A situation where they're only allowed to do it if they have 10,000 separate and unnecessary antennas is absurd.
Personally, I think it's ridiculous that a TV station can broadcast their signal free of charge to anyone capable of receiving it, but if a company wants to receive it and then pass it along to somebody, they have to pay a fee. But the problem is with the ridiculous law, not with a completely reasonable interpretation of it.