The bar entertainer isn't really even there to get the attention of the audience--if the audience doesn't want to pay attention, the entertainer has to find a way to play in the background.If the audience wants to sing, well, the entertainer has to do that.
Playing in bars is a job, and, if you need to work, you find a way to do your job. If you don't need the work, you find some other way to spend your time.