He spent a lot of time and energy making clear that he wasn't racist in his election campaign due to slurs and people making light of previous comments made that perhaps hadn't been fully thought through at the time. In light of that, this seems to be a rather strange decision - what better way to prove his credentials than to loudly support the biggest anti-racism festival around.
Moreover, he's lost some major sponsors for this years event (with just a month to go) - Unison and Sertuc in particular pulling out - and one of the big draws for larger acts, the overall message. Moreover, Richard Barnbrook (the BNP member of the GLA) has already proclaimed this to be a significant victory on his part.
So, what do you think of this? Does it show short sightedness of the work that music can do in helping raise awareness and build bridges or is there a valid point about political agendas at such events?