I'd just tell the host when you're arriving that you're just there for the music/ambiance and not looking for food or drink, if you really don't want to buy anything. I'd try to arrive earlier in the evening when it's not full. They also have plenty of food/snack options, so I think it would generally be polite to order *something* when entering any bar/restaurant, even if they have other reasons you may want to be there.