They probably designed the car with Europe regulations in mind first, and then rather than try and design an entirely new brake light for the US just took the lazy/cheap way and lit up the other two available light bars.
But presumably those would be too small for US rules so they lit additional segments, making the arrow, which is what the parent commenter referred to.
I've seen various Tesla models that just use a single horizontal line of LEDs (either amber or red on color) for their turn signals in the US.