If people want a crosshair, they'll make one. With thread or a pasty on the monitor, some kind of overlay, mod, idk whatever. A potential problem with an option in the menu is that it's significantly easier, and many would not go through the trouble to "get" a crosshair otherwise.
What bothers me personally about this is that for me playing without a crosshair added a lot to the fun and unique experience which KF1 was.
I feel like if there is a crosshair and it's on by default, many who are new to KF will leave it on without thinking about it, never even exploring the awesomeness which is iron sights only.
I feel like it should be off by default (if the option must remain) and almost feel like if you go to turn it on there should be an "ok" dialog box saying something to the effect of "Turning on the crosshair detracts significantly from the KF experience. Try playing without a crosshair - you might just like it a lot more! [ OK ]"
Forcing it off above normal sounds nice as well. I don't think the crowd that having a crosshair will appeal to would be playing much above normal anyway. I could be wrong about that though.