You need to remember who is liable here. Everything going on is to do with who will end up paying, given VA is in VA and doesn't have much cash left.
When a chargeback is successful, if the money can't be recovered from the company (VA in this case), then the institution that provided the merchant facilities to the company is the one that ends up paying. For Amex and Diners, they both acquirer and issuer (acquire merchants, and issue cards). Hence any successful chargeback granted by Amex they will probably end up out of pocket.
For VISA and MC, the issuer and acquirer are often different. If the bank that issued you your card is not also the one providing the merchant facilities to VA, then your bank will not be out of pocket if the chargeback is approved - it will be VA's bank.
With the obvious differences in exposures, some institutions are more likely to be diligent in checking the validity of a chargeback (checking actual eligibility) than those that have nothing to lose.
In the case of VA tickets, this is where people are in a bit of a bind - and also why the administrators were so keen to get court approval to issue "credits" for which they were not liable. The tickets purchased were for flights - subject to the contract and conditions of carriage. In the case of the VA flights, the contract provided that in the event of cancellation, a credit voucher would be provided - and this applies to cancellations by the customer or by VA (a big difference to the CoC for QF - where refunds are to occur in the event of cancellation by QF).
So for VA tickets, if a credit is issued by VA, there is no breach of the contract for the provision of service as that was what was agreed up front. With no breach of the contract, then there is no failure to supply, and no charge back right - as yet. This is why the Administrators were keen to be able to issue the credits - as if they did not do that, then the contract was breached, and thus charge back rights would be available. However, if the credits were issued, and they were liable for them, then that would obviously be an issue - hence the need for the court decision.
It would appear that the position of those issuers that there is no breach due to provision of a credit is technically correct (even if it is an interpretation only being applied by those facing losing the money). If the credit is then not honoured, then things might change. What would become really problematic is if new owners decide to put conditions on honouring credits (as is very likely). If the "credits" are "honoured" but with a condition like "credit voucher can only be used for a maximum of 10% of the value of the airfare" (so that they get 90% paid in cash), and all the fares are bumped up so that they are 20% more expensive across the board, have the "credits" been honoured. Technically they are redeemable, they have not been discounted at all, but in reality, they are effectively worthless, but would a charge back be available ?