Blaming puritanical opposition to fun seems like a 'just so' narrative that fits some of the apparent facts, but is contradicted by others. For instance, American culture glorifies caffeine, particularly coca cola which is portrayed in corporate propaganda as a 'fun' drink. When it comes to commercial products like that, there seems to be no hint of an anti-fun bias, ...except perhaps in the Mormons.

While anti-fun attitudes probably play some role, I think it mostly comes down to a pro-productivity mindset. Caffeine is generally understood to enhance productivity, and for that reason may as well be a revered god in the American pantheon. Alcohol is borderline; it's a known quantity that generally seems to leave productivity intact except in cases of alcoholism (alcoholism is consequently seen as shameful). Alcohol bans have been attempted, and are still intact today in remote regions of Alaska where the productivity impairment alcohol induces is seen as a particularly concerning matter. Cannabis remains maligned because it's seen as making people lazy and content, not because it might be fun. And amphetamines are legalized in circumstances where doctors say it will cure productivity impairing disorders.