My understand is that when you purchase a gift voucher you become a creditor to HMV. Creditors should always be paid from largest value of money owed to the smallest. By temporarily refusing gift cards HMV are simply restoring the correct creditor order, so that the creditor with most to lose doesn't do so at the expense of other creditors
That's a made-up system though, made up by banks probably.
In general the public should not be expected to take the same risk in dealing with a company as other companies might have to take.
It's bad for business for one, and also unreasonable to expect a member of the public to have to vet the viability of every company they deal with when only small value transactions are involved.