this can only work in United states. Where it is your right to shoot anything any if you're standing on the pavment in public. In singapore police will be called unless you are freaking famous or doing it for charity =p
It can be done pretty much any where in the world.
Just don't publish an image without a person's consent
It may not be morally right but legally yeah no problems.
It will be consider a chargable offense when there is a breach of peace, if the photographer took a picture and do not cause any breach of peace, it's not an chargable offense. The only time he may get into trouble is if he publish those photos, and ignores subject plead to remove them when discovered, then the "victim" will be able to lodge an magistrate complaint. He cannot be charged for public nuisance or breach of peace, because the subject was unaware he was being photographed at the point of time.
I believe experience street photographer like him can use wide and flash here, and get away with it. You may be surprise how some of these photographer gets away with shots usually taken at 28mm but it's actually not difficult to understand.
i attended some journalism talk at SPH regarding STOMP once. apparently if it's in a public place, you have all legal rights to take a photo of ANYONE, which is why STOMP is legal.