That would be true if the scope had the same size objective as it's exit pupil. But, with most rifle scopes, the objective is far larger than the scope's exit pupil. Thus the light hitting eye looks alot brighter simply because it's concentrated in a smaller area.
You are correct in that a paper tube would transmit 100% of the availiable light whereas even the very best rifle scopes do loose some of the transmited light. E
