Maybe it comes from the fact that a jump shot requires jumping, whereas it is explicitly forbidden to take your toes off the ground when you attempt a free throw? :)
No, ou are not forbidden to take your toes off the ground when you attempt a free throw. You can jump if you want its just that its easier to soot free throws without jumping and jump shots are harder to hit.
I think too that FT and JS are different but I could say hat a player with very good range are better free throw shooters. So I think JR is better compliment than JS for free throws.