I suspect the explanations are more fundamental than that. Societally, we're supposed to ignore poor people.
Were there many seats in that room? Is the architecture designed so people bubble in and out, with no space to contemplate?
[WS:] That can be easily debunked. If you go to places like, say, Venice beach in Los Angeles, there are many people performing in an environment that Is not conducive for ideal viewing of performing arts. Some of the performers are crappy, other are pretty good. The good ones attract crowds (based on my casual observations, as many as 30-50 spectators), the crappy ones do not.
This demonstrates that: (i) poor people are not 'ignored" as you allege; (ii) physical environment does not deter spectators from stopping and viewing the performance.
What makes the difference between these two situations is the framing or the context - Venice Beach is an "institutionalized" venue for street performers, L'Enfant metro station is not. This is what the article is trying to explain.
There is another point that I would like to add to it - the marketing/entertainment industry and the electronic gizmos they employ turned music (and art in general) into background noise, basically a nuisance that people ignore or block off. This has nothing to do with the quality of performance but wit human perception. Sex and orgasm is very stimulating and desirable, if experienced sporadically and in a right setting - but if you turn it into a "quickie" or even a forcible rape - it turns into nuisance or nightmare. This is precisely what the entertainment industry, with the help of car radios, ghetto blasters, and ipods, did to music and art - it turned it from orgasm to rape by constantly bombarding the audience with it to boost sales.
Final point - you observation about the choice of the genre (classical music) is a good one. Such choice increases the probability of failure (i.e. no-one noticing) simply because most people (I am among them) have relatively little exposure to classical music and thus do not have the right cognitive skills to spot virtuoso performance right away - at least not without additional props, which were lacking. But most people would not recognize "good performance" in any system - from cars and computers, to construction, architecture, etc. - without additional clues. That is why we have experts.
Wojtek