It is against the law (at least in the US, not sure about European countries) to display favoritism to one creditor vs another when you know the company is insolvent.
I doubt this would apply even if the law holds where Instawallet is based. The money in the accounts wasn't being "borrowed" by Instawallet, so depositors aren't strictly creditors any more than if I left a dog at a boarding kennel while I go on vacation that I'm a creditor to the kennel.