...the book is dedicated to Sirhan Sirhan, implies rather more than is actually the case.

The book is dedicated to "Harriet Tubman and John Brown, To All Who continue to fight" and also "To all the political prisoners in America", accompanied by a list several hundred names in tiny print. And Sirhan Sirhan is included as one of them.

As Carol said there, it seems likely that whoever put that list together may well have believed Sirhan Sirhan wasn't responsible for Bobby Kennedy's murder. Who knows? 1968 was a very strange time, and so was 1974 when that book was printed.