If you're paying for entrance to the gig then I don't think there's any obligation to buy a drink as well, just because its in a pub - you wouldn't feel you had to buy a drink at a gig in a theatre, for example.

But the original posting was about an arrangement made between a landlord and and a group of folkies, who reneged on the deal. The landlord could have thrown them out - he chose not to, but he won't offer again. But why didn't the organisers explain the arrangement to their members beforehand, or tell them to stop eating and drinking their own supplies when they started tucking in?

Without the arrangement, these people would have had to pay to hire a room, or practice in the street. A cup of coffee and a sandwich sounds cheap enough to me.