Except that they aren't all free to use in that fashion. Again, I don't want you to think that I'm shit talking or anything, but those themes come under different licenses. For example, here's the I feel dirty license and what you can do with the theme:
http://studio.st/i-feel-dirty/.
Others may be under simliar licenses, or they could be completely free to use in any fashion you choose. It's a case by case kind of thing.
Edit: double post
