It actually works the other way round in some festivals, especially with the larger pubs, as they sponsor events in the festival financially, may give out big jugs of beer to the musicians' table, free hot food in the evenings (big trays of sausages, chicken wings and chips!) For that they get advertising in the programme and official onstage thanks at events they've sponsored, and, hopefully, a greater footfall.
Sure, the smaller pubs can't afford to do this, but just putting " Musicians Welcome" signs in the window will give some clue as to whether sessions are OK there, and hence bring more folk in.