What is up with the spiderman movie producers and casting red headed actresses as blondes and vice versa?

I always wondered with the new spiderman movie coming why the hell do the producers keep casting red headed actresses as blonde which we could see with Brice Dallas as Gwen Stacy who is clearly a blonde and now Emma Stone who is also a natural red head as a blonde yet again for Gwen Stacy in the new movie. Also for Kristen Dunst case she's a blonde and they got her for a red head. Wouldn't it be easier just to have the red heads play the red heads and the blondes play the blondes. Doesn't anyone else agree with me that we'd rather see Emma Stone as Maryjane cause she looks the part but also that if she were to be MJ we wouldn't have to be sad when her character gets killed off in the sequel to the reboot spiderman movie where they introduce the new Green Goblin which will follow the comic closely.