I don't think it's stupidity either but they think there are a whole lot of quilter's that would like to have the boring work done for them...
Me, I always wash fabric when I bring it home. I don't like the smell of it or the feel. I want to know what it's going to be like after it's laundered. To me, the sizing and dye take away from the real fabric.
I think fabric sizing is a way of hiding something...washing reveals the real fabric.. the quality.