Because this fork in the road is above the horizontal (east-west) line. Generally speaking, you can allow the right turn and the horizontal (east-west) through train to be released at the same time (of course, it is different if there are non-motor vehicle lanes). If you are going to turn right to the northeast fork in the road, you must cross the east-west through train, which will cause a crossing line and be dangerous.
If you want to enter the fork at the east (right) intersection, you should drive in the right-turn lane of the east (right) because the fork does not cross the north-south straight line (up and down).
Related content explanation
Both straight and one-way street belong to indicator signs and are one of the main signs in traffic signs. Used to instruct vehicles and pedestrians to drive in the specified direction and place. The color of the indicator is blue background and white pattern; Shapes are divided into circles, rectangles and squares.
This sign means that only all vehicles can turn left (or right). It is located in an appropriate position in front of the intersection where vehicles must turn left (or right). When there are special regulations such as time and vehicle type, auxiliary signs or additional patterns should be adopted.