If they sat up straight, the belts would be tracking across the collarbone, like they're supposed to.
But booster seats put them up higher and creates a greater risk of hitting the backs of the front seats.
There's a short buckle strap that pulls the shoulder component down to the waist belt so that the shoulder belt fits a medium-sized kid (or a small adult) properly. The National Highway Safety Board doesn't recommend them because they're not tested . . . but so long as the strap is strong enough and fits properly, why not?
But do kids ever sit up straight in the car? Mine do the same thing, lean on the arm rest, usually toward the center so that they can bug each other as much as possible.