I do think it's customary for the guest to pay for meal ashore so the real question is about the invitation. Seems like a miscommunication somewhere since you haven't seen other complaints in their reviews. I guess if other guests mostly do full board they wouldn't have this experience. I'd guess the miscommunication started back with the initial booking and the half/full board conversation. Just guessing....

No comment on the alcohol topic as I don't even drink when I run my own boat.