There seems no other explanation for this "Belsen was a gas" name for an Evil Twin jacket, other than it being a pun on Bergen-Belsen, a concentration camp where approximately 50,000 people were murdered. (And, not that it makes much of a difference, didn't even contain gas chambers.) But hey, it's on sale! Update: Yes, a Sex Pistols reference; that's no excuse.
Even if that was a Sex Pistols song (and I DO love the Sex Pistols)....if they had a song called "I Love Genocide" or "Isn't Racism Fun?" I doubt they would be naming clothing after those songs. This is title is like that, but just with another layer of education over it. You have to know what Bergen-Belsen is...which most people should IMO....as a Jew and a human.