The WRC is changing their rules to a spec that's closer to the S2000 class (NOT the honda), rather than the GroiupN class they've been using. Suzuki and Subaru are saying this is the reason they're leaving the WRC. However, this article is likely closer to the truth.
The good news is, this is just he Manufacturer pulling out. It doesn't mean there won't be Subarus in Rally. They'll still dominate RallyAmerica, and with the new S2000 style cars, it's more likely independant groups will race in the WRC and bring their own cars. So you may still see Subarus in the WRC.
Remember, it's not Subaru that modifies the cars for the WRC, it was Prodrive. As for Rally America, Cosworth builds those cars.