First of all, it relies on the incorrect assumption that 'the same people' would pay a lot of money to see him play at a concert hall. I'm not sure this is right. About one in 1000 people did stop and recognize him, and I feel this fraction is not too different from the actual percentage of people who have heard of him and would pay to see him, so this isn't surprising. He is famous, but not that famous.
Second, there is a fundamental misunderstanding on what it means to go to a concert. The whole point of the concert is to take time specifically to listen to music in an attentive and dedicated way. It's not like an improved version of listening to music in my headphones during my commute, it's a wholly different experience with a different purpose. Moreover, it's a collective ritual. We don't go to the concert hall just because of the music and because of Joshua Bell, we also go because it's nice to treat yourself to an evening out, to dress nicely, and to enjoy the fancy atmosphere of the theatre with its, say, baroque frescoes or modern architectural design. Joshua Bell in that context is literally not the same as Joshua Bell in a subway, otherwise why would he dress nicely for the concert? And you know what, there is nothing wrong with that. Theatre and music have always been this way.