Banksy has been putting up art, fighting the man, and being an inspirational activist for years. This is why.
Interestingly, Banksy lifted a great deal of this quote from an essay Sean Tejaratchi wrote in Crap Hound Magazine way back in 1999. Banksy may not have credited the essay very well, but is the re-appropriation of the text validated by the content? In any case, Sean Tejaratchi seems pretty okay with it being a cool dude and all.