I wonder about the timing of this. ADV once said that these days, their involvement starts even during pre-production. Then why don't the companies announce that they hold the license even before the series starts? That will stop the (honorable) fansubbing groups.
Let's face it: fansubs may eat away from the market for anime (why buy when you can download?), but it may also generate interest into new series (why buy a DVD when you don't know if you'll like it?). I think the best example for this has been Noir: it generated a huge buzz in the fansubbing community, and the ADV DVD releases were highly anticipated.
I don't recall having seen sales numbers for that series, it would be interesting to compare those with a comparable series that didn't get such a long run in the fansubbing community.
Also, the plot of anime series can kick in pretty late. With Witch Hunter Robin, for instance, the plot only started moving around episode twelve. With four episodes per disc, that means that you only get a hint of the overall plot at the last episode of the third disc. At a MSRP of USD 30, that means you spent USD 90 before the story kicks in. Before that moment, you can stop buying discs without missing a single thing (because it's episodic before that point anyway).
I don't have all the information, but I would hazard a guess that the anime publishers use fansubs as a very cheap marketing instrument. Keep tabs on the fansub community (there are enough sites out there that list the new fansubs being released), allow the fansubs of your properties to progress until the plot kicks in, and then announce your license/send C&D letters. Now you have an audience that knows there is a story, but they can't get it for free. The only option open to them (if they want to know what happens next) is to buy your DVDs...
Obviously, I don't have all the information, so I'm pretty sure these guesses are completely wrong.