Redemption Song is an over long self-indulgent book. Salewicz never misses an opportunity to mention what great mates he and Strummer were and any criticism of St. Joe is voiced in the most timid and understanding way. Strummer comes out of it as not very likable (which clearly wasn't the books intention), and it never gets to grips with what it was about him (more so than The Clash collectively) that inspired such devotion.
As for the violence thing - well in 1976 it was almost impossible for pop music to discuss the role of violence in effecting political change. Pop music which thought about politics at all (sweeping generalisation coming up) had long taken an uncritical pacifist stance. The suggestion that violence might sometimes be justified was a rebellion against the orthodoxy of a "counter culture" which by 1976 was compromised into irrelevance.
(Also, tormenting hippies by talking about violence was much less likely to get you a good kicking than doing it to Teds or Skinheads)