doPayment vs doDirectPayment and $0 payments - regression, bug or feature?

As per https://github.com/iATSPayments/com.iatspayments.civicrm/issues/362 it appears that the switch from doDirectPayment to doPayment can result in $0 transactions being sent to payment processors.

@mattwire has kindly provided some code to handle the issue in this extension, but I wonder if it was intentional, and if so why, and if it should be documented somewhere.

Or whether doPayment should be smarter?

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information