Some of them were in disguise so that they couldn't be recognized on TV. They fear that their style could be recognized from other perhaps not so legal locations.
John and I walked by there today and to check it out and saw they used some kind of foil. So in fact, the graffitis were done somewere else and then glued on the Tate Modern... maybe the only possiblity to create such a big piece of art in short time.
