It is illegal because it is a pirate site, which hijacks the stream from the legit, licensed providers, like ESPN, ABC, or whoever. They do stream games, illegally, and make money from selling their own advertising usually via banner ads and scrolls.
There really is no gray area here, and it is no different than downloading a current theatrical film that someone recorded on their cell phone.
We can debate whether it SHOULD be illegal, and copyright, the direction of media and open-source, but that is a large topic, probably not suitable here.
In any case, it is illegal to stream a game you are not licensed for. Usually, the consumer does not get prosecuted, but schools and networks go to extreme efforts to shut down illegal streams. Often, they are caught and closed mid-game. I can see the allure, for people who have no other option, and YOU won't get in trouble, probably, but it hurts the networks and indirectly harms the schools.
Maybe a better example. In the past, NU made good money for our PPV games. I bought almost all of them. It was another revenue stream and helped athletics. The site that is now removed from your OP might also have that game. For free, even. For free for us, that is. But, NU would never see any money from us watching via these pirated sites. They would lose money that I would otherwise have contributed to the school via my PPV purchase.
I hope that helps.
Another good example is the Longhorn Network. To me, that one is unconscionable, as a fan. I'm sure plenty of Texas fans will be watching illegal streams, which also hurts their school. In that case, though, the LHN isn't available widely, even with their new deal. I suppose it's a bit more understandable, as a fan, because there literally are no options for many of their fans. But it is still, also, illegal.
