Fansubbing is illegal. Period. Distributing it is copyright infringement. The only reason anime corporations let them do this, is because it drums up interest for their shows, and usually about the only way to do it overseas, since their advertising campaigns are non-existent. This is why no fansub group that gets a C&D will ever fight it in court, because they know it's completely and entirely illegal.
The anime companies are giving Crunchyroll permission to distribute because the DVD market is on a steady decline, and there's no online distribution system in place. They didn't really tap into Steam or XBL. Even if they did, people could just get the same thing for free. Crunchyroll doesn't do downloads because they need people to watch the ads. If they let people just download the episode, there's room to remove the ads, and distribute again. Anime companies are making the money off the advertising. Because if they don't, they won't be making money at all.
They need to force everyone who watches the ep to watch the ads. People are probably only going to buy the show on DVD if they've seen it. Anime companies aren't big, you'd almost say adversive to 2 things: advertising in North America, and taking risks. So they're not going to advertise a show unless it's big, but if it can't get big without advertising....
To answer Eilistrae's question, there's a lot of reasons. First of all, I don't know how a company would feel about recruiting from people who steal from them. Another is location, fansubs you can do at home anywhere, but they just don't live near a company that works on it, and can't move to said location. Another is money. Fansubs completely demolished the DVD market for anime, so they can't hire the same number/quality of people. Which pushes more people to fansubs, because even if the quality isn't there, it's free.
Maybe 5-10 years ago, when people actually cared about quality in their fansubs, it might have been a good idea if circumstances allowed, but now, when people only care about speed, it'd be a disaster. And they probably wouldn't last long if they had to do it as a job, rather as a hobby or something. Crunchyroll-type sites are pretty much the last thing left for anime companies to do, and mostly because of fansubs. I'd like to see all the fansub groups get sued for damages, and have the companies build up some decent advertising, or something like that.
If you're downloading fansubs that are being streamed legitimately on websites, you want anime to die.