People in the left turn lane who stop well past the line. You can't turn until you get a green light anyway, so why do you need your back wheels past the stop line. It's not like the people who can turn right on a red light who scoot past the stop line to get a better view to the left.
It all depends on conditions. Maybe the left-turning lane is too short and if you don't use up all available space, the line of cars wanting to turn left will end up clogging the "fast" lane on the highway.