It's always been the case that offers bring "new" customers (Though many simply swap between Sky and Virgin)
All the companies do it and let's say it brings in 10,000 new customers during the offer period and Virgin buy the netflix vouchers at face value that's £36 pounds per customer that's £360,000 in costs to Virgin if they extended the offer to all customers with a TV package with a Tivo and lets say that's about 3 million customers (guess) that's £108,000000 quite an expense for no perceivable gain unless they made everyone sign new 18 month contracts to get the offer