Just to let you know, there are some people who actually hate retweets. I would prefer that someone I follow would tweet only in their own voice even if they're resharing what someone else shared. As you said, he signs his retweets with "credit via". I mean what more do you want him to do? In my experience the most annoying thing on Twitter is just because I'm following Marc Andreesen I have to consume all the random tweets he retweets from his friends/colleagues/interns/acquaintances/some random guy he met online.