I noticed this too and contacted Apple via chat to find out if I could exchange my White Summit Nike sport loop for the black one and the person told me there weren't any exchanges available for me. Wonder if that's because the new Nike loops aren't on sale yet.
So, it sounds like it's specific to the type of watch, meaning Nike band only for Nike watch? I'm hoping they'll be able to exchange the black sport loop for a different color, but it won't be a big deal if they can't. I have the pink sand sport loop, and I'm sure I'll buy another band if they can't swap out the black one.