you not being force to get their products, you are free to use any other provider of service that meets your requiment.
making things cheaper when yoou buy more is standard everywhere, be in 2 for 3 or reduce unit price when you 10 ext, or even a certain % off when you spend so much.
Offering internet at a cheaper price when you take anopther service follows that same logic if you buying 2 service off me I have 2 cance to make prodift, therefore instead of wanting to make 10 profit per user per product I will happily only make 7£ but as you buying 2 service that mean £14 profit from you.
However If i was inchard I would offer single product but at a mark up, i.e. if you don't tike out line rental add £2.50 to the monthly fee if you don't take our call plan at £5. Bit like sky currently does for their ADSL