Yeah, but it's only a risk if there's been a complaint, and the landlord's been warned about it, and persists, which most times never arises. It's a lot less than the risk involved in having a lock-in, which all good landlords seem to do from time to time.
If musicians told to stop playing in a pub moved outside and played on the pavement or whatsoever, I can't see that the landlord could be held responsible. I suppose if hey took their drink is outside a case could be made against him for serving them, going against his licence renewal, so we mighthave to play thirsty for a bit. But apart from that any law-breaking would be just down to the musicians.