I live in NYC and have never heard that.
That sounds like another conspiracy theory urban legend.
Do you have a source on that?
I find impossible to believe that a gang or some thugs would even dream of pulling of a stunt and being more successful then multi-billion dollar fashion companies so they could go shoplifting.
It would be more plausable if it was leather jackets or something, but white t-shirts?
Re the white t-shirt thing...it's been the standard summer fare in my 'hood for the past 5 years too. I always figured it was because when you get arrested the local jail will only let you keep a plain white t-shirt and those jump suits are itchy I've been told.