The Gold Card area was extended at the start of this year and now includes Birmingham. Virgin would not want their passengers to get 34% discounts when travelling from London to Birmingham.
In my view a better workaround would have been to say that the GC discount is not available north of Milton Keynes on Virgin, at least keeping consistency with the Network Railcard.
Quite why VTEC (actually the deed was done when EC were in charge) pulled out is beyond me. It only affects London-Stevenage journeys anyway...