Hi Greg,
I believe the practice of using Volume Up/Down inputs on soundcard interfaces started with Allstar looking for a way to detect CTCSS and CoS signals from the radio.
I wouldn't call it either a software or a hardware defect, it is a "hack" in the meaning of re-purposing an existing feature for something else entirely. It works well when done properly but it can have unintended consequences.
73,
Thomas
KK6FPP