That's because he doesn't shoot that many FT's. His true shooting percentage is basically your average shooting efficiency for an NBA player. Which means he isn't having a terribly inefficient season. It's easy to just look at raw FG attempts.
Your beloved Rudy Gay averages 18 points on 17 shots, so do you consider him an awfully inefficient player?
yeah I remember saying DeRozan, Bargs, and Gay are all inefficient and Champ(possibly JM) disagreeing with me.
and a guy that doesn't get to the free throw line is inefficient. you don't just say he doesn't get to the line so his numbers aren't that bad, getting to the line is part of a scorer's role as it makes them more efficient. there's a reason they're called FREE throws for everyone except Dwight and there's a reason teams will foul him allowing him to get 1 point per possession as that's below the league average in terms of points per possession.