I can think of a few reasons why that might be challenging.
1) on a road with high speed traffic it can be challenging to get over to where you "should" be, especially if the road is up hill and you're going 10 while traffic is going 45+. I will absolutely stay on the right in that situation, as much as I'd rather not, but I'll also sit there and let the turning traffic turn before attempting to cross the street. If you do go straight, as you should, cars also have an ugly tendency to go by you in the straight lane and then right hook you to get in the turn lane, even if you are riding the way you are supposed to be riding. There's a lot of room in your drawing but there isn't always that much room so it can be complicated.
2) What do you do when there is a right then lane, and then a combo right turn / straight lane? I generally go for the middle of the two lanes and go straight but it's perfectly conceivable that a car could right hook me at that intersection, so I will usually stop in front of all the cars (or behind them) so as to either be more visible or it doesn't matter.
If you're riding on streets and intersections don't make you as nervous as a long-tailed cat in a room full of rocking chairs then you're half asleep. That site Alice linked is a good refresher read.