It's simple. If you came into a Nike Town, we didn't want you leaving without a race bib for the Human Race. And while sure we could have just given them away, that would have only resulted in hundreds of race bibs immediately making their way to the nearest trash receptacle. So instead we turned them into fashion. Runners got their bibs, and the trash man got off nice and easy.
Get a bag, get a bib. Your "bib" is automatically activated at the register.