I think it's advertising. People with iBooks are likely to take it on the road, where people can see it and go 'ooo I want one, what is it? Oh it's an Apple'. IMacs are generally used at home or schools so there's noone to sell to...
Works both ways too; when my powerbook backlight goes to sleep and I'm sitting in front of a bright light source (window) I can see a faint backward apple logo on my black powerbook screen.