Something tells me most people aren't running and buying scenery for both.
There's no need to use both sims at the same time, you might also be one of the many users which instead are (or were) running FS9, then would like to switch to FSX, perhaps when upgrading your system (sooner or later, it will happen), and since we don't sell the two versions separately, switching to FSX will not require to repurchase any of our products.
That's the main advantage of selling the scenery under the same license, and it's far more common that those running two sims at the same time. We are trying to ease the transition towards FSX, but selling separately is a way to make it harder than it should and, basically, it's a way to commercially exploit this unfortunate market division.
There's no technical reason why an additional version should cost double since the biggest part of developing a scenery is not spent to create two versions, but modeling/painting.