Imagine a child on a standing lane who needs to sit down to range find.
He is tethered to the rifle on a short leash. He has to get down, then up again. Then perhaps a whistle goes and he has to wait. He cant put the rifle down.
He forgets the range, sits down again, and stands again. he can't put the rifle down because it's is tethered to him.
The rules as they stand render the use of SP slings impracticable and in my opinion dangerous.