It's just the fashion industry digging through the trends and fashions of the past 60 years for things to recycle. It doesn't have to make aesthetic or semantic sense; it just has to sell. Leaning on existing fashion from the past lends a certain sense of legitimacy to whatever trend they are pushing, so it's a tried and true tactic to reintroduce them — conveniently stripped of any social classifiers associated with it (but they do look great on Instabook).

That said, today's teenagers wearing Nirvana T-shirts may smell like teen spirit, but I doubt any of them has even heard a single song of that band. It's a bit weird.