I don't have any problem with the guy personally, I think he is ethically justified, considering his poverty background. However I don't think this is a tactic that should be emulated by well-educated entrepreneurs. Breaking important social rules to make profit is rarely beneficial on the whole.
Is this covering the US copyright position too?
In the UK, if they're performing on the street they'd need a street performance license (local authority) and a music license to cover the copyright of the writers (from PRS). You might need a second license if you were imitating a particular performance (from PPL).
This appears to contradict the spirit of your comment if not the letter. The GP stating that singing wasn't allowed "on the subway" and you replying that it explicitly was allowed "in the subway". I can see how you can both be right and individually agree with your link too though so the above is really just for clarity.