I can't speak to Oregon law, below I've pasted California Vehicle Code Section 21202 with relevant sections emphasized.

Here is a picture of an intersection I ride thru frequently:

https://www.google.com/maps/@33.6754.../data=!3m1!1e3

If I got there before the blue car, I would be directly in front of it. If I got there after I would be directly behind it. There is not enough room for me to stand to the right of the blue car and allow a right turning car enough room to safely pass to turn, but just enough room to temp someone to try, potentially clipping me. Also if I were to to the right of the blue car, once traffic started moving I would be vulnerable to the blue car or others in the right lane making a last minute decision to turn right. By being in the center of the lane I don't have those problems. Once the light turns green, I am well to the right of the thru lane by the time I reach the middle of the intersection and the straight thru traffic is passing me safely.
-------------------------------------------------------------------------------------------------------------------------------------------
21202. A. Any person operating a bicycle upon a roadway at a speed less than the normal speed of traffic moving in the same direction at that time shall ride as close as practicable to the right-hand curb or edge of the roadway except under any of the following situations:

1. When overtaking and passing another bicycle or vehicle proceeding in the same direction.
2. When preparing for a left turn at an intersection or into a private road or driveway.
3. When reasonably necessary to avoid conditions (including, but not limited to, fixed or moving objects, vehicles, bicycles, pedestrians, animals, surface hazards, or substandard width lanes) that make it unsafe to continue along the right-hand curb or edge, subject to the provisions of Section 21656. For purposes of this section, a "substandard width lane" is a lane that is too narrow for a bicycle and a vehicle to travel safely side by side within the lane.
4. When approaching a place where a right turn is authorized.

—California Department of Motor Vehicles, CVC 21202, Operation on Roadway[SUP][5][/SUP]