As a non-drinker I'm often baffled as to why I can enjoy a night at a gig without boozing but others are really deterred if they can't have a drink.

Habit. Expectations. Habitual expectations. (Also nerves for performers.)

Beer and folk music are inextricably linked in some people's minds; maybe that's another problem.

That was partly my point; we have a booze-and-folk culture, extending to the celebration of drink and drunkenness in song. Perhaps there is, or was, a slice of the folk club audience for whom giving up the clubs was easier than giving up the beer.