Actually, now that I've been ruminating it over
I have arrived at the following conclusion: those who were meant to enjoy it, did. If you read the long Washington Post version, you'll see that a few passers-by did stop, namely those that were connoisseurs: they recognized the piece, the virtuosity or the violinist. So it did make sense for them.
I posted the story because I shamefacedly have to admit that I would not have stopped. One I have never heard of.... (even now I have to check his name) Joshua Bell, and I would definitely not recognized the Bach. Now had it been some more mainstream names, like Beethoven, Mozart, Chopin, Vivaldi... Those tunes would have chimed differently with me, and I dare say the less sophisticated commuters of that train station. So maybe the experiment was unfortunately aimed at a very narrow audience.
Still, the question stands, and I believe that at least once in our lives, everyone walks by a miracle (or more) and misses out on an opportunity to enrich their lives or at least their spirituality.