This just goes to show how deluded people are about what we can actually perceive and what we assume is true based on cues around us. There are obviously some people at a violin concert who really know the violin and would have been able to identify Bell's superior skill in the subway but they are a vanishing minority. Even then they might have thought nothing of it unless they recognized him in particular since they are on their way somewhere else - time is of the essence. Nearly every other person passing by couldn't possibly tell how good Bell actually is and even if they could there is no reason to assume they are going to be dumping $100 bills into his violin case nor making a scene. So what if some busker is absolutely awesome at the violin?
The other thing that gets me is the assumption that we pay a lot for fancy performances specifically based on the quality of the performance. The experience of a show is composed of many things including the experience of dressing up, anticipating the show ahead of time and being in a crowd of people who appreciate that particular form of expression. If I strictly want to hear Bell play the violin I can listen to that any time by hitting up YouTube or a music store - just hearing it is nothing special. The special thing is the whole experience. I guarantee if people knew Bell was playing it would have been a huge thing because people would have set aside the time for it and expected to be part of a big, excited crowd with common interests.
More than anything I was amused at the surprise when parents dragged their children away from the subway station performance. Have these people ever tried going anywhere on public transit with children, or taken any long trip for that matter? The absolute last thing you want is the kid in question dithering over yet another random thing on the way while fussing that they are tired and want to be there already. Encourage Elli to listen to a violin performance when I am at home? Sure! Encourage her to stop and park next to a random busker? Unlikely!