That's pretty much the definition of a gig, isn't it? Just about every gig I've ever played has been at a bar, a restaurant, a club, or whatever. If it's worth it to me (Work:Fun+Money) to play the gig, I play it. Whether someone else is making money at the same event is a separate variable and not necessarily relevant.
For example, we played a gig for tips at a restaurant/beer garden last night. We brought in a pretty good crowd and the restaurant had a better than average night. Good for them, and I am not going to hold their feet to the fire to get some of that money. It's a new and struggling place, and it's their livelihood. It's not mine.
They want us back, understandably. It's a symbiotic relationship; they get the benefit of selling food and drinks to the folks we bring in and we get a stage to get our (mostly original) music in front of an audience. It's a win-win.