This has been discussed on Watchdog recently. They believe it unfair as well, and have therefore provided a downloadable template you can send to BT to complain.
I must warn you though that they have also reported that those that have complained so far have had no joy.
Personally I dont see the problem - what is the difference between discounting for those that do, or charging those that dont? We both up paying the same amount whichever way they package it.
Dave