In the Young Bucks interview with Meltzer they said the Hot Topic CEO was in Orlando during WM this year, saw Bullet Club shirts everywhere and wanted to put them in the stores. So he went to WWE and when they said they weren't theirs, he got in touch with the Bucks and that's how the shirts got in Hot Topic.
I don't much care for most of these guys matches but for them to have such a hot shirt without WWE is pretty groundbreaking. Makes me excited for the future of wrestling.