> Now... for a regulated system of tokens (eg currency) a developer can’t do that: they are legally bound to accept fiat currency even though it is not redeemable for any “real” equivalent (eg gold).

This isn't true in general. Legal tender status is very loosely an obligation to accept for existing debts, but it is not an obligation to accept it as, say, a means of accepting a offer. So unless they are giving you goods first, and the looking for payment (which certainly is not the norm for digital goods), they are under no obligation to accept currency.