Because you should have not been offered that upgrade it the first place. As I've said in another
thread the £12 upgrade was only supposed to be give to customers who:
1) had other services
2) had an upgrade on one of those services (ie TV L to XL or Phone M to L or XL)
If neither of those conditions applied, the offer should not have been given.
However one of the billing systems (the ex-ntl standalone broadband one) would allow you to apply the discount without those conditions being met.
As part of the billing migration these accounts are being identified and the incorrrectly offered discount removed.
The discount did not recontract and could be removed at any point after 30 days if the official conditions ceased to be met (i.e someone downgraded TV XL to L etc)
In future all discounts offered by Virgin have strict rules built into them to specifically stop this sort of thing happening.
It was the agent that you spoke to that did it wrong, not VM themselves